Well, let me first say that you are, in a way, bring up the lined versus unlined debate, which - IMHO - shouldn't really be a debate to begin with. Every player who has played or does play fretless probably has his or her own preference over lined or unlined - and as such, every preference is just as valid as the next. It's what works *for you*, not what you think everyone else should do. For me, I'm a unlined player. I had a lined fretless, it didn't work out for me, and so I want an unlined fretless now. A lot of it has to do with the fact I'm also an orchestral upright bass player. Unlined "feels like home" to me. So, when you ask, why do basses even have lines or dots - it's because some people like them, though they may not necessarily be right for you.
Now, I have a few reasons that I prefer unlined. When I had a lined bass, I was drawn to look at the lines because I felt it would help me play in tune. This was much in the way that I *cannot* NOT read subtitles in movies, even when it's the film, not some words, that I actually want to see. I also think that when I play an unlined fretless, it forces my ears, not my eyes, to make the decisions of how I finger the notes. And like I said before, I play double bass in an orchestra - unlined just feels "right" to me.
As for your question, yes, in my opinion, playing the dark will probably help improve your intonation. BUT, I think that it will improve your intonation because it will improve your "listening intonation," not because it'll magically make your fingers fall right on the semi-tones.
I say try it. If you're not happy with how you're currently progress (I'm also assuming you have a lined fretless) or you just think you could try progressing more quickly, give it a shot. It can't hurt, right? Alternatively, you can get one of those sleep eye covers and practice with that on.
Recently, I played a Fender Custom Shop Jaco Pastorius Signature Jazz Bass. With the epoxy fingerboard and roundwound strings, this thing just SANG. BIG, full, yet singing tone - I loved it. But, while I was playing, I had to forcibly close my eyes so as to not look at the lined fingerboard. Really, it's *my* problem, not the bass's, but for me, the next fretless I buy will be an unlined fretless bass. I also prefer the look.
