I agree on your description of her as a one off or special or unique and different talent. But the sad thing is that many of these one offs seem to tend toward self destruction. Why is this true? Also you mentioned she is back on the booze. I guess we have heard that story before.
Another person I have always wondered about is Tim Curry who was in the British production of Rocky Horror Picture show before he starred in the film. That Frankenfurter character he plays is probably the most bizarre a character in a film that I have seen. His Sweet Transvestite song or number is probably one of the most far out or unusual numbers ever in a musical. I got assigned that role of Frankenfurter in a college production of Rocky Horror picture show. It was a very tough role for me. I was a woman playing a man imitating a woman. I had to dress in the corset and do that freaky facial makeup too and try to imitate Curry's accent and voice as the director didn't want me to lip sync it. Tim used his DVD camera to tape me doing the Sweet Transvestite number and I put it on my youtube page because there are so many fans of Rocky Horror Picture show.
But it was just a fun and different role for me. Tim Curry seems to define this role. Is he a bizarre character off screen? A boozer or worse? He seems to me like a guy who is "out there" in the same way Amy Winehouse is.
I guess you have to say Bix sure is a one off talent too and his struggles with booze and early death happened. But like some posters here have said the music is what matters.