All BS aside, I do see from a lot of lined board players that they don't feel like they could play an unlined board. I don't believe they would have much problem with it, after a short adjustment period.
I've never played a lined fretless outside of a dealer's showroom. When I decided to buy my first fretless, I bought it unlined because I really do feel like lines make you look like you can't play without lines, and I'm just insecure enough that that matters to me. Plus, I like the connection with traditional stringed instruments. I was actually a bit scared, because I was afraid my intonation would be awful and it would take forever to learn, but I doubt it was more than a few days before I sounded just fine.
Not having owned a lined fretless, I do wonder if it might be a distraction when shifting back and forth between fretted and fretless.
I do kind of like the look of partial lines on some instruments, like the ones that are just little tick marks and aren't all in a row. It can look pretty cool if well done, like the cascading position markers on an NS Design instrument.
And yes, both of my fretless instruments do have side dots, and it's true that upright basses generally do not. However, upright basses also generally stay at a more or less fixed height off the ground, making it much easier to learn that type of muscle memory to hit the same positions more consistently. Bass guitars are all over the place, so I think side dots are appropriate, but I feel like now, after 15 years of playing fretless, I could probably manage without the dots, too, if I really had to.