For us, we just don't generally see the point. We get in line together. We might save steps by not all going to park the stroller (when we were using the stroller), but the non-parkers will just wait by the line.
The times someone has had to leave the line to use the bathroom in my family, we just all leave the line, or the bathroom-needer would just skip the ride that time. I don't think we've ever done the latter, though.
Of course our decisions are also managed by the fact that we're not tiny people. I know DH won't mind me saying this, but if he gets out of a line (or tries to squeeze forward), he is impacting EVERYONE, big time. If he had to exit Peter Pan for instance, well I'm not sure it would be possible. So it's not really an *option* for us.
DS once had a sudden urge at the end of the Casey Jr line. Since they have the turnstiles (point of no return) and I'm short and won't be climbing over railings, we just asked the CM to be let out. She specifically told us to come back to the exit and she'd let us back in, but the line was actually short, and we felt no need to do that. We just got back in line once the bathroom had been visited.
In theory, in lines that have an easy exit area, like the chain at TSMM or a gate at Jungle Cruise, the bathroom-user would get out there, the other adult would stay at that point, letting people go by and explaining what's going on, then the user(s) would come back to that point. But I know we never used the Cruise option, and cannot remember if we've used the TSMM option. And the reality for us is...it's just not THAT important that we all stay together, or that we ride any given ride at that exact moment. We don't want to cause discomfort by squeezing by people. And since we're a party of 3, with two adults that aren't tiny (though I have lost 42 lbs and DH has lost 30 since Feb so we're workin' on it!), one of us is generally riding either entirely alone or in front or back of the other two anyway! So the idea of being separated by many cars isn't *that* different anyway.
All that means: we don't see why people do it, and will always think "why is it THAT important to all be together, and if it is that important, why not just wait to get in line until you're together?" when people do managed to squeeze by. (and in thinking about it, it's a RARE line indeed that anyone has tried to squeeze past if DH is with us...I doubt it's ever been tried in
Disneyland's lines, especially Fantasyland)