I'm with Sean on this one - as a second round in the day, particularly if it follows a good liquid lunch, which is always welcome in the winter, it has a lot to recommend it.
My club is one of those Sean refers to where fourball play is only permitted after lunch on weekends (and that is a recent development - which caused some old members to resign
for the rest it is a two-ball course.
I think Bob is right when he says the clubs who play a lot of foursomes do it that way because they have always done it that way. But if you press a bit harder, I think there are two other factors:
First, foursomes golf is played mostly at quite traditional courses. The ethos of these courses derives in part, I would think, from Oxford and Cambridge who play all their competitive golf in the winter months (hence the President's Putter at Rye in the early days of January). Summer is supposed to be for exams - and then holidays.
Second, these clubs tend to have older memberships and I think foursomes recommends itself more to older players who are not so desperate to flog their own ball and like the camraderie of foursomes.
For myself, I increasingly enjoy foursomes, by way of variation. For anyone who says that the true measure of a person is found in how they behave on a golf course, that truth is multiplied many times when it comes to foursomes. It is one thing to keep your composure when screwing up your own round, but something else altogether when your partner is screwing up your round.