And to add further chance of damage are the salts that remain in the leather from the tanning process.
Some of these are slightly hygroscopic and tend to attract and hold moisture.
Not if it's the right kind of leather it doesn't. Vegetable tanned leather shouldn't have any remaining "salts," and shouldn't hurt the gun, especially if it's been finished with an acrylic such as Fiebing's Resolene.
I've kept a blued S&W model 10, and half a dozen other guns (blued, parkerized, etc.) in their holsters for years. Not a spot of rust, and I don't oil them down as frequently as I probably should. Haven't pulled some of them out of the holsters for months, except to look for rust - nope, so I put them back.
I recall someone recently with a stainless SIG who used a kydex holster, and it showed rust after sitting in the holster for seven hours.
Nylon holsters are just as capable of retaining water as raw leather, and far more capable of retaining water than properly finished holster leather. I've seen more guns ruined by being put in nylon holsters or gun cases after the lightest sprinkling of rain...
As to why keep them in the holster? Simple - all my handguns are loaded, and the safe way to store a loaded handgun is in a holster. I never have to worry about whether a gun is loaded or unloaded - I don't have to administratively handle (load/unload) my weapons before carrying or when I get home. I do a quick chamber check before heading out, and that's it.