I'm just saying, people start cover bands, they rarely get rich off of them, and for the most part they aren't great (I can think of a couple that are ok, like badfish, but in general it's a big field and not that impressive). However when you're playing in those sort of bands you do get a lot of experience about how music is constructed and how the process and industry operates. Battery might not be "as 'good'" as Metallica is "good," but these people do not resort to art snobbery or bust down on every open mic when people try to karaoke "Enter Sandman."
I'd even go so far as to extend this into the realm of visual art forgery (although I know you said it was not about copyrights, but I wished to offer my finger to the modern western methodology of understanding, let alone enforcing intellectual property rights one last time), it isn't as though a discerning eye will think that a forgery of the Mona Lisa is the real thing, or that a cover of "Sunday Bloody Sunday" sounds the same as U2's version, but while experimenting with these premade and often beloved environments it is possible to learn something about their place in our culture and also about the mechanics of their operation in ways that can help people who are novices better understand songcraft/prose/drawing. When you reject their efforts out of art snobbery you deny their advancement, you prevent them from learning. Don't you know that it's a fool who plays it cool by making the world a little colder? I'm not saying that art snobbery has no place, or that an extremely high standard shouldn't be established or sought after, but I think that when you apply it to people who are still learning the mechanics of storytelling or their chosen artform that you seriously hinder the capacity of your artform to interact with society and promote meaningful connections between people.
Criticism is always positive, but out of hand rejection never is.