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.....
1944 Mauser byf K98 8mm, K31 7.5 x 55 Swiss, Rem M700 Classic 8mm, 1920s Colt New Service .45 LC, Browning Hi-Power and HK P7 in 9mm, SAIGA .308 22", SKS 7.62x39mm, S&W 686 Plus 6" and Taurus 617 2" in .357 Mag, Ruger MkIII Target 5.5" BB .22LR, Mossberg 500 & Marlin 120 in 12 GA, etc.