There isn't any difference in tone or playability due to fret lines. As James says, they are a crutch, or maybe just a convenience. In my case, the lines are there (faintly) because it's a modified fretted board with the slots filled with veneer...they're not very visible, especially since I stand to play bass and can't really see the face of the fingerboard anyway. If you've had good finger placement (sometimes called "planting") and technique, the switch to fretless isn't that difficult. I personally prefer fretless for both sound/tone and playability. My playing is more busy, fluid, soulfull, and inspired as there are no frets to get in the way.
It's probably just me, but I've never understood why fretless basses aren't more popular.
Peace, Mooh.