I've never seen a fretless guitar, but the only time I've ever heard a bass line that I liked it was on an electric bass, fretless. Why are bass lines generally so boring, I heard some wonderful music from that thing, even though it was electric and my tastes are acoustic. I've been considering taking the frets out of my mandolin so I can try some of that, but that WILL wait a while. Probably I'll learn fiddle first, and then if that goes well remove the frets. (I'm not stuipd enough to mess with any insterment in that way unless I'm sure of what I'm doing)