Apparently you're reading more into my comment than was there. I never said anyone had to or should not play an instrument with a certain amount of strings. My point is simply that the phenomena of guitar players watching bass players' hands and getting thrown off by the "extra" string is something that can actually happen. It has happened with me, in those circumstances I mentioned.Then they need to adjust. What if the guitarist had a better handle on the song but preferred a 7 or 8 string guitar? Should I say, no you must use a 6?
Agreed. But are most of us here in that situation? But that's sort of logical. I also wouldn't turn up to a job interview on site for a concreter's job with an electricians tools. Even if I don't prefer those instruments, I would play them if that was what was needed for the gig.
If I was playing with a guitar player at a jam who was playing a seven string guitar, I might very well be thrown off by by watching his hands. That doesn't mean I should switch basses or that he should switch guitars. It just means the visual cues we sometimes rely on are, in these cases, not accurate.

, unless it was technically impossible to play on 6...which is actually possible. Some of the licks can only be played authentically with open strings.