In my playing, I use visual cues for 2 things:
1. visualizing the notes on the fingerboard (mental map).
2. providing an approximate location of where to play in-tune on the fingerboard.
Lined fretless basses provide me no useful help for any of these 2 goals. There several reasons for that:
- the lines are located on the front side of the fingerboard, and they are out of my sight in my normal playing posture: in order to see them I would have to either tilt the bass upward, which puts some strain on the left hand, or to bend forward akwardly. What I can see very well, though, are the side dots (more on this below).
- concerning the usefulness of lines for visualizing the layout of the notes on the fingerboards, lines (assuming I would see them) are not good for me because there are too many of them: with one line per chromatic note, it is easy to make a confusion between them (eg. 8 th and the 9th lines can be easily be confused with each other). Side dots located at the positions of lines 3, 5, 7, 9, 12, 15, 17, 19, 21, 24 (with double dots on 12 and 24), on the other hand provide, a clear map of the fingerboard, and are perfect for this purpose.
- unfortunately, presumably because the bulk of the market for lined fretless basses are bassists who mostly play (or started on) fretted bass, makers place the side dots on their lined fretless basses in the same place as they would on a fretted bass, i.e., in the middle between 2 lines: thus those side dots are of no help at all to me for intonation, and just through me off badly.
- fortunately, on unlined fretless basses, makers place the side dots exactly on the position of the (absent) lines, which is right they should be if one wants to use them as visual cues for intonation. That is why unlined fretless basses are better for me than lined ones, both for orientation and for intonation. If there would be lined fretless basses with side dots located at the lines, they would be fine for me (I would use the side dots and ignore happily the lines that I anyhow don't see). As far as the "missing dots" are concerned, with a proper hand shape, it is not a problem for me to interpolate between existing dots (though I find that the "double" dots on 12 and 24 are suboptimal in this respect: it would be better to have these "on top of each other", instead of "side-by-side" as they usually are, as they would provide a more accurate visual cue of the octave).
Now some precision on how I use the side dots as an assistance for intonation: as I mentioned above, any visual cue (either lines or dots) provide at best an approximate location of where to play to be in-tune (even with properly set-up bridge); this is due to physical difference between a wide and soft finger versus a sharp and hard fret. The fine-tuning has to be obtained by a combination of ear-training, and muscle memory, together with a very reliable hand and body posture. Also, it is important to include in one's playing open strings and harmonics, which help in monitoring the accuracy of intonation. Practicing some dedicated exercises with closed eyes (I am not talking about performing) also helps sharpening the earing accuracy and build-up the muscle memory.
I also stress that this is what works for me (primarily double bassist, having never touched a fretted bass), and it is clear that others may prefer lined fretless basses for many good reasons. I see no point about claiming that one way or the other would be better in general.