Don, I don't think anyone is saying musicianship ruins or detracts from singing in general. I think the general gist of the posts is that for some of us playing an instrument is not an easy flow of the fingers. We have to pay close attention to what we are doing with our hands and that distracts us from our voices.Add to that the position some of us are in when we are playing. I tend to lean forward to watch my left hand on the guitar neck. This is not a good position from which to sing. Consequently my voice suffers and it is my strongest instrument.
Plus I think a lot of people agree with you that the individual song dictates the need for instrumentation.