This. You want to get someone into Linux, show them what Compiz (or KWin) can do.
Seriously though, I just wouldn't even bother trying to talk about it; it's their computer, their choice…the most I might say is "I just like it because it interests me more than Windows", and this would be completely honest. I don't see a need to get into semantics about licensing or personal morals or whatever (I personally don't put that much weight on it myself :-\).