With very good reason - I've slipped a little but not fallen so far. I love my pair but will have to consider rubber heels at some point because of this.
Regarding the whole Topy issue, I'd like to share experiences and ideas relating to all the issues surrounding sole guards.
Balance
I discovered when I first bought C&J shoes with the heels packed full of nails just like Loake, Church, etc, that after the leather on the heel toplift wore away, I slipped on the nails (I pretty badly scratched some Handgrades on a pebbly concrete hill). Hence, I got a Topy put over the heel. So you could Topy Heel + Sole for maintaining 100% balance. Then you don't have to worry if the sole guard is 1.8 or 3 mm.
Grip
Topy will get slicker with wear, but still better than leather. Very few will disagree. However, you must remember that the waist has no grip. I have slipped on some steps in a Topied shoe because I became lazy and though I was wearing sneakers hence walked of the waist and then slipped. Of course I would have slipped on the waist if there was no Topy, but I would have been more careful if the soles were all leather.
All Rubber vs Combination Heels
Combination looks better when new. I think sense must prevail here and you should only have one surface for a particular part of the foot. If you watch motor racing, they spin when the grip suddenly changes. I believe there is a potential for this to happen with combination heels. The heels with a dozen nails are dangerous for slipping as above, so you can just Topy over these.
Comfort
I believe the Topy should be more comfortable, not just for the often cited reason as a cushion for the walk. They can stop the shoe from curling up during wear, stopping the toe puff from digging into the foot. They do almost half of the shoe trees job already.
Breathability
Sometimes I feel that my Blake shoes breath through the soles, other times I don't. I would say it isn't a big issue. The sole leather is tanned to stop water going in, I doubt that it blocks water in one direction and lets it pass out in the other direction. The cork used in a Goodyear Welt is designed for insulation purposes likewise. I don't the shoe will rot because of a Topy. Others disagree.
Flexibility
After a few wears with the Topy, I believe that the shoe will flex just the same as before. You could wear the shoes a few times without Topy first or break them in on carpet. I'd personally break them in first so the sole moulds to my foot shape better, then apply Topy with no possible ill effect. Some disagree.
Structural Integrity
I don't believe that a Topy will damage the shoes integrity. A partial re-sole might however.
Durability + Wet Weather
Topy, probably lasts even longer than dainite, though I don't have personal experience of this. Topy 3 mm vs Oak Bark 8 mm is probably 3:1 durability in dry wear. Leather soles (even Oak Bark) get badly worn even in slightly damp footpaths. Topy all the way here. You could wax polish the waist for water proofing. Particularly for girls' shoes, I believe they are 100% necessary, if I had shoes with 3 mm leather soles I'd kill them after two wears!
Looks
Topy adds a touch of thickness to the waist, but they also stop the edge in-particular the toe from getting chunked up and needing to be polished and smoothed after every wear. Topy'd shoes look nicer than dainite IMHO. You'll see that the shoe has a Topy while walking, and the black Topy might not complement the rest of the shoe. Some, including myself, don't like this very much, and some even 'judge' others for such soles. That's taking the whole thing too far. We need to retrain our tolerance to sole guards :). You can get biege and dark brown Topy's too. The repairer should grind the edges at the time of application so they should be almost invisible when standing. Judge at your discretion.
Topy Elysee vs Vibram
I heard from a cobbler that they tried Vibram sole guards and the glue came unstuck in summer.
Walking Surfaces
I believe that pavements in Australia may be slipperier than those in UK, and especially modern shopping malls with marble floors are not designed with leather soled shoes in mind.
Topy vs Metal Tips
Topy will not scratch, and you won't slip or even worse trip going down stairs/slope. At some sporting events like the Australian Open, they do random metal scans and it would be a hard to convince security why you have metal tips on your shoes, particularly as canned drinks are banned for their ability to be used as a weapon. Crazy I know.
Time of Application
Best do to while they sole is still hard and hasn't been exposed to anything wet. Perhaps if you wear a few mm off the sole first they will stand out less, but they may make the applied Topy look not quite as even. If you Topy over a rubber leather combination heel you can preserve the original heel toplift, colour of the original heel and balance better.
If you Topy earlier, it will preserve the contours of the last in sole better, this may be good or bad. Breaking shoes in at home on carpet or in public could cause injury. For instance, I was walking down a tiled hill in the Sydney CBD with new shoes on and had to stop quickly as a van pulled out of a lane while I was still on the footpath. I stopped fine, but it left me thinking.
Resole
Cobblers will resole Blake or Goodyear Welted shoes with either leather half or full soles. I believe the change to shape of Blake shoe shouldn't be a big issue as the upper is often glued to a small layer of leather to which the out sole is stitched. The upper should therefore be retained in place during the resole. You'll never find a cobbler with Oak Bark leather in Australia. The cost of sending to B. Nelson or the manufacturer takes too long and is too expensive and they may not like doing it. The Topy won't last anywhere as long if the sole is already soft, but it will still last a fair while. I don't know about Topying over a shoe with a leather half sole, sounds a bit dodgy to me. One issue not often raised with a Goodyear shoe is the eventual shift in the cork bead and slipped gemming, on the one hand Topying may make repair difficult, but on the other hand perhaps early repair will prevent these issues to a large degree? A leather half sole may be 55-60 vs 40-50 for a Topy sole, but considering a 3:1 wear ratio for the Topy, the price difference increases over time. Topying a Goodyear shoe with an extremely worn sole won't fix any cork bedding issues. Most manufacturers won't refurbish more than twice, and I would forget resoling with leather more than 2-3 times. The ability to Topy might also be gone if it was done with a leather half sole. A beveled, fiddleback waist or decoration on the sole edges will be better preserved with a Topy.
Far less chance of things going wrong with a Topy, and its cheaper, longer lasting and less hastle.
Repairer's Perspective
Even the highly competent ones stress it because overall it is better for you and for them. Almost all repairers should be able to apply sole guards well, but I'd still take them to a good one as there isn't much of a cost difference. The price doesn't differ between suburbs and CBD in my experience as well.
Dancing
You can't dance, or maybe even walk, like Fred Astaire or a ballroom dancer with rubber. But I'd say the only places with enough room to dance properly these days would be serious latin/swing dance clubs where you could change your shoes perhaps? Many venues have polished concrete dance floors which are very dangerous. And do you really want a partner to step on expensive shoes? If you are coming down a grand staircase, you can't glide down it as easily and effortlessly with rubber. On the other hand, Federer moves very smoothly in sneakers.
Multiple shoes
If you have multiple pairs of a particular style of shoe, like the plain captoe oxford, brown brogue or sharp looking penny loafers, you could Topy the older pair when you get the second so you have wet weather options and don't have to compromise on style. If your going out for a few drinks I think rubber soles are more sensible :).
Why not just buy rubber soled shoes in the first place?
The range of dainite soled shoes are far fewer than leather soled options. Topy looks better than dainite IMHO and probably lasts longer. Shoe factories often use a roller to shape the sole to the shape of the last, so it wouldn't be practical to make a rubber mould for a full rubber sole for every last in every size.
Morality??
I like to be old fashioned in many ways. I would feel like part of me died a bit Topying a new pair of shoes, so I don't do it that often. However, I then check the leather soles for hardness after every wear, so it would save me a bit of stress Topying them straight away.
In summary, its very much head versus heart. If you use your head you'd Topy before wearing them or after single digit non-wet wears. Shoe will last longer, be safer and be (a lot) less costly to repair in the long run. But I haven't quite gotten over my prejudice against Topies yet. I hope we all can someday.
Hope this helps and thanks for reading.
. You can get biege and dark brown Topy's too. The repairer should grind the edges at the time of application so they should be almost invisible when standing. Judge at your discretion.. 
