Hi,
Hard to say ... I think that many singers, are more poetry addicts than they are musicians, and folks like Bob Dylan, Roy Harper, even Peter Hammill, all sing what they feel and most of the music is about helping define what they say.
As such, I think of them more as actors, than I do musicians, although I sincerely doubt that either of them do not know music at all ... they probably do better than we think.
But a singer, has to have some kind of musical ability, and it comes in the area of timing, however, if the music is made to support the words themselves (think Peter Hammill or Kate Bush), I tend to think that they define the music around what they wish to say, not the beat, or anything else. Thus, I suppose, that one could say that some of them are not musicians.
I won't discuss the rock'n'roll side of things as I do think that many singers are not musicians, but have a good feel for what they are doing, and while you and I can think this is cheating, it works in most rock music on the radio .. in what I know call MOS!