Let me just add a little more to my take on this issue. I don't think that care of shoe linings is necessary or desirable every time we polish our shoes. However--as I now consider this more--once in a while still seems like a good idea--maybe once a year. By treating the lining leather with a conditioner, you are not introducing the kind of moisture that is problematic--water and perspiration. In other words, not all moisture is bad. If leather conditioner had the same ill effects as water and salt water, then applying it to the outer leather would cause problems too, but, of course, it doesn't. Moisturizing the lining leather is simply keeping it supple so that it never cracks or develops fissures from having dried out. If you apply a light coat of conditioner once in a while, rub it in really thoroughly, and lastly wipe off all moist residue, you will have lining leather that's dry to the touch and will remain that way, but that, in addition, has been kept supple and new-looking. I have had the linings of old shoes get dried out and a little tatty-looking from lack of care. Since this isn't visible when the shoes are worn, and since it would never keep one from being able to wear the shoes, we overlook it. Nonetheless, this kind of care certainly can't hurt. To those who say, in effect, "I can't be bothered" or "it's overkill," I would say "fine," but that's a different argument than one concerning maintenance advantages. Everyone is free to treat his possessions as he likes, and if it's too much trouble, don't do it. With very expensive shoes in particular, however, I'm concerned about giving them the very best care I can.
Oh, just as an aside: lee_44106, the reason we use shoe trees is not to keep the insides dry. It is to maintain the shape of the shoes between wearings and prevent them from curling and taking on misshapen form. The issue of whether or not shoe trees actually absorb any moisture has been exhaustively debated several times on this forum and AAAC, and the consensus that has emerged is that they really don't. If they did, I'm sure that Edward Green, Vass, John Lobb, et al., would provide unfinished more-absorbent cedar (or another softwood) trees, but they don't; instead they provide varnished (often hardwood) trees that couldn't possibly absorb any moisture.