For hunters, or real operators whose lives may depend on it, camo painting clearly makes sense.
Black, is a color that doesn't appear in nature- anywhere...
Other than those applications, for just "personalizing" a weapon, why not?
I've done a half-dozen rifles in Krylon, and a few more (and a handgun) in Duracoat. The Krylon, while not as durable, actually holds up pretty well- use a heavy matte clear coat over for protection- and if you decide to change colors, or pattern on a whim, it's easily done.