Two things, for me: flexibility, and the lovely noise - especially from a P-style pup.
Flexibility because it can do anything a fretted can (though it's quite a bit harder, for, say, tapping), plus slides, trapping harmonics, etc.. (Not that I personally can do these things, you understand )
Practicing on fretless improves my fretted intonation, BTW.
I recommend the Yamaha RBX200F as a starter, if your defret goes badly.
(Also the unlined board can look very very cool, if that floats your boat. Example: [url="http://basschat.co.uk/topic/165919-veillette-paris-fretless-4-price-reduced-again-l1200/page__hl__veillette%20paris"]http://basschat.co.u...illette%20paris[/url] . And I don't even like blue basses... )