I can only think of one trad band of any stature that does not sing, and that is Lunasa. And while I could listen to them play for days, I can relate to the sentiment that "it all starts sounding the same" -- that's what it was like for me while I was getting acquainted with Irish dance music.

It seems clear that if you have a capable singer you will appeal to larger audiences. You would have to be quite a dedicated tune purist to turn down that prospect.

Cheers,
Michael