Humidity and moss and substrate, oh my

[xdeus]

Is there a very good reason of somesort that noone uses moss as a substrate? Aside from a possible messiness factor? Because it seems to me it holds moisture and looks lovely. Naturally, constantly wet moss would be a bad idea, but otherwise... ?

I think two factors that make it unpopular are cost and the ability to spot clean easily. If those aren't a problem, I don't see why you shouldn't be able to use it if it isn't kept too moist.