Susan, seems to me there's good reason for Harpgirl's suggestion:
1. HearMe as commonly practiced has great gaps of silence while only the keyboards chatter, and whoever may be trying to act as host vainly tries to get someone to get up and sing;
2. Being "Host" is consequently a difficult and more or less thankless job. If the room is full, it's hard to keep track of who has sung/played, who is only listening, and who only needs encouragement. An hour at a time is probably as much as one should ask of a host, in my opinion.
3. Having an expectation of doing three pieces might get participants to prepare more thoroughly, instead of just showing up and flopping about until the oppressive silence gets around their unprepared bashfulness.
Now, then, therefore and quinsecontly, having a host with a limited term of office, so to speak, and encouraging (if not actually requiring) a three-piece set from a performer would be easier on the host of the moment, tend to keep the time filled with song, and maybe even raise the standard of performance.