I find, speaking JUST for myself, that it's better tto separate tune sessions and song sessions. If the tunes keep flowing, the players get much more into the swing of things than if there's a stop- start. And in my experience, a significant proportion of the singers resent the time given over to tunes- and vice versa. And session "leaders" are useful because they (or in the case of a festival, the organisers) have done the necessary sounding out of the venue, negotiation over place and time, and (should be) there on cue to launch things off, and hopefully to give anyone turning up some idea of the broad parameters- e.g. it might be Irish, Old Timey, Bluegrass, French, English, mixed etc., it can be concentrating on favourites or trying new tunes, it can be turns-about-the-room or as inspiration takes- and a good session leader will have an instinct for what the participants want.