When there are frequent intersections and rest breaks, sure. Otherwise, it seems like people do get pretty spread out if everyone rides at their comfort speed without regard to staying in a pack. Though I suppose if there were pre-specified stopping places that everyone knew ahead of time, it wouldn't matter if people got spread out, and there wouldn't be the stress of being the person at the back who's lost sight of everyone *and* has no idea where/when the next random stop will happen. (Tricky to manage on an unfamiliar route, though.)

The other difficulty with the catchup game is that the person going slower gets shorter rest breaks than the person going fast, but probably needs longer ones/more of them (if the slower person has been been trying at all to follow the group, i.e. pushing past their default speed).

I think terrain makes a big difference, too. I'm not actually thinking of hilliness, though obviously that has effects, but rather of traffic etc. Some routes offer more or fewer convenient places to stop; also some make it easy to ride side by side and/or pass, while others don't.