I agree that there's no comparasion between a pop artist and a theatrical vocalist or any true vocalist. I can't really think of anyone in the pop field who compares to the Jason Danieleys or Audra McDonalds of the theatre. But they are also two entirely different types of music and performers.
However, the beauty of theatre is that there's a place and something for everyone. It's non-discriminatory in the entertainment of its audience. A musical doesn't have to be of a more traditional flavor like My Fair Lady, Curtains, or Corem Boy (which I thought was wonderful). It can be Pop or Rock like Rocky Horror, Mamma Mia, Jersey Boys, Xanadu, or Legally Blonde.
I enjoy plays as well as musicals and the local park playhouse productions in the summer sun are a blast! In the end, this person is only denying himself the full richness of theatre's live performance by limiting his experiences to plays. If that's what he wants - so be it. Me, well I'm greedy -- I want the whole pie, not just a piece.