I think saying that someone cannot take a song, and sing it a little differently without calling it their own work is not a good idea. If it weren't for artistic impression being allowed as a whole, then you'd of never had many songs done in their current fashion, such as "Walk this Way" from Aerosmith.
disclaimer: I don't currently listen to Aerosmith or any other secular music artist for that matter, but I used to.
Also, a singer using artistic expression to sing/play a song differently than the way it was originally written can show that they do have true artistic talent. Of course, that goes with any form of "art", I suppose. If it weren't for artistic expression, we'd of never seen Davinci's works, Michaelangelo's, nor Vincent Van Gogh's.
Also, as already alluded to, the allowing of artistic expression in America goes a long way toward showing the true meaning of freedom. If musicians weren't allowed to sing the American Anthem, or any other song, the way they wanted to, or felt best, on a particular occasion, then that would represent a more communistic government, I'd say.
That said, there are times when artistic expression isn't the best means of delivering any media. I think, specifically, of times when you are directing music. Sure, the director can hold out notes a little longer, or do things like that, but if you're say directing a group for the first time - think church music director, directing the congregation - then artistic expression in that sense is more likely to cause confusion than to do any good.
But if you're singing/painting/drawing/scupting/speaking, whatever, and you are not responsible for directing others at that particular moment, then there's no threat of confusion, I think, and therefore you should have a little bit of freedom to change things.
Of course, that does not mean you should be able to recite a poem, and change words at random, or particularly give historical information, and randomly change facts as you see fit.
I talked too much. I need some coffee.
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"If to err is human, then I must be some kind of human!" -Me