For me, it's both aesthetics as well as functional, but I'm not categorically against them. I think they're a great thing for beginners, but, just as with fretted instruments, you should (eventually) be able to play without looking at your hands, final intonation should always be by ear, and IMHO, line material in the board just adds risk of fingerboard level inconsistency eventually.I, for one, never understood the bias towards a non-lined fretless board. I'm all for anything that will help me sound better. I know Angus was kidding, but I've heard arguments about this.![]()
Not looking at your hands is really useful for reading charts if you need to, or simply putting on a better visual performance on stage if you aren't using charts. Also, on intonation, the lines should just be there to help you build muscle memory for getting "generally" to a note (especially in the upper registers where it even matters how much your finger squishes and/or rolls slightly more or less.)
Back in the 80's I was doing a lot more fretless electric (jazz combo, fusion, etc.), and I only had lines for a while because I had a bass defretted... however, once I got comfy on it, I was all about losing those lines on its replacement... and never missed them. The side dots were all I ever occasionally needed at that point.
IMHO/IME