well i can hear what you guys are saying, but it's not just artsy telling of an interesting story, it's also a bio. and as such, i would think what it should also portray is why was this particular person so big in his/her respective field. why did they make a difference? why were they so fascinating to people?
with Johnny Cash, a big part of it was his voice and the way he performed his (and others') songs. that doesn't come across in that movie for me, at all. and it makes a huge part of his story that's missing.
with Stevie Nicks it's somewhat similar - a big part of her popularity is her raspy sexy voice and the way she dresses. they way she dresses is obviously easy to replicate. but if the actress portraying her is actually singing the songs and her voice is just regular middle-of-the-road voice, that would be a big miss in any portrayal of SN.
with Lindsey's voice doesn't matter that much, to me, as his voice is nothing special and many people can replicate it. a good actor should also be able to portray his typical moves when playing. and Bradley Cooper can easily display very similar sexiness (plus naturally blue eyes) that is essential Lindsey.