You cannot beat the sex appeal of fine leather, which is a reward to behold (and smell) unto itself even without your favorite gun inside, but it is unkind to blued finishes. Which is more valuable, your gun or you holster? I use nylon for most day to day purposes, and especially any situation where I may draw often, although I have more than one holster for most.
An obvious drawback to leather holsters is the temptation to go high end and get one that is as expensive as a cheap gun. That never happens with nylon of course.....
Mauser Werke, Schmidt-Rubin, Colt, Walther, HK, Weatherby, Browning, Ruger, Savage, Bushmaster, Marlin, Mossberg, Saiga, S&W, Sig Sauer....a few friends of mine