Personally, I think singers need to do what's right for them. If it's also right for me I might buy their record or attend their concert. If it's not I'll give it a miss. By and large, I personally prefer people to sound more-or-less like themselves, but if they're good at what they do when trying to sound like somone else, my doubts can be transcended. What I do know is that if I was a singer and some jumped up scrote tried to tell me what I could and couldn't do, my natural instinct would be to do the opposite of what they wanted. I also have no problem at all with - for example - skinny white English boys singing the blues. Wouldn't particularly want to listen to it even if you paid me, though!