Yes, I've heard John Kirkpatrick's observation about the habit of playing "medleys" of tunes for a dance. He claims that it used to be normal to stick to one tune throughout a dance, and than when 78 rpm recordings began, they wanted to offer more variety. A 32-bar tune (English, Scottish, or Irish) played at a normal dance tempo will take about half a minute, so you can fit in three tunes each played twice through (or two played three times?) onto one side of a 78 record. Three minutes also became the standard length for a popular song, for the same reason.

Modern English bands vary in their approach to this - some will keep playing one tune throughout a dance, but if they do this they'll often add some variation in the later rounds. More commonly they'll change tune about halfway through, possibly more than once. Scottish (RSCD) dances are almost always danced to medleys of three or more tunes, usually with key changes.I don't know what the custom is with Irish social dancing now.

English session players (and Irish sessions in England) have followed the pattern, most often playing sets of two tunes, each played three times through. We had a fiddler from Orkney at our (English) session and she tended to change tunes after twice through, which confused us. We also found it less satisfactory, because when you hear a less familiar tune, you need to hear it at least three times to find your way into it.

The reasons for all this may well be as JK says, but I also think there's something about a three-minute song that seemse very natural. With the exception of long story ballads, most songs can say what they have to say, lyrically and musically, in about three minutes. I'm not sure it's the same with dance. It's simply that dancers are now used to a change in pattern after several minutes on one tune. Session players, too, just like a bit of variety, and I can't see that most of them would want to play one tune more than 3 or 4 times - hence the two-tune habit.

Marje