Whatever the group's going to do, it's important that everybody does it.
Damon can see over the heads of the rest of the group - I can't; in a group I'm totally dependent on the rest telling me what's coming. I can't see hazards, I can't see oncoming traffic and I can't see the top of the hill! So I may have all my faculties, but that doesn't mean I don't benefit from some collective responsibility.
On TRAT, although the group riding skills were generally very good, people wouldn't pass a call up and down the line - so one person would shout something incoherent and the other end of the group wouldn't hear it. We also had one who freely admitted to being scared of riding two abreast and one who wasn't scared (
) but had safety issues with it, so whenever there was a call to double up, we'd end up with two twos and then a little line of four ducklings still in single file. I don't think it matters particularly which you do, but it's really important that everybody does it.