There are some interesting opinions at http://www.styleforum.net/t/179041/my-first-shoes-with-topy-put-on/0_30. In particular, I found this quite interesting given that Nick is from B Nelson.
This debate has been discussed many times in this forum. It really comes down to personal preference.
Some no matter what, prefer walking on and the look of leather. Breath-ability has no impact. Nor does flexibility or balance. I never lost a debate with a maker regarding said.
So, these comments regard economics only:
I am using averages here. Everybody's different.
Generally, sole guards last twice as long as a 9-10 iron leather sole. Assume it takes you a year to wear out a pair of leather soles. If you were using sole guards it would take two years to wear them out. Factor in the cost of replacing the leather soles and time if you are sending them back to the maker. Maybe bringing them to a shoddy repair shop that may screw them up. Compare that to every other year replacing the sole guard at less than half the price of a new sole. Also almost any repair shop can do this without error. Do the math, if a shoe lasts 20 years what is the difference in terms of the cost factor? Add the other benefits of sole guards. To me the overall value is incomparable.