Yep.
One very big advantage.
They don't turn green when you keep them in storage for 20 years. Also, for some reason, when brass or copper is kept in the vicinity of leather, it "greens up" about ten times faster. It doesnt' even have to be touching the leather. Knives with brass pins and such in the handles will grow green fuzz if kept in a leather sheath. In fact, if they are merely in the same box as a piece of leather, they will grow green fuzz.
Same with bullets.
And holsters tend to be made of leather. So there ya go. Use nickel plated cartridges if you plan on keeping a gun for a long long time loaded in a leather holster and almost never ever shoot it.