My preference is also for mixed sessions, as I both play and sing, and our local monthly ones do try to be all-inclusive, regardless of age, standard of playing etc, and move between tunes and songs and then back again after 2 or 3 of each.
It can be frustrating the first time at a festival new to you to know where's the best place to head for a session, and you invariably find out that the really good one happened yesterday in the place you weren't at.
To that end, the festival I help organise, does have a session listing, so that visitors do know where to head for singing, mixed, instrumental, bluegrass, etc, rather than just relying on sessions to run themselves. We do put in a session leader, but it is always emphasised to them that it's not their concert, but rather to ensure the session gets started, and then to make sure that everyone gets a fair crack of the whip, and no-one dominates it. And by special request, we are running a "tunes only" one this year.