Some people just cannot do math. If there are 17 people in a group, and there are 4 singers...no matter how good.... who each feel they cannot go more than 4-5 songs without doing another, it soon becomes evident that some are going to get few, if any, turns. Thus, I prefer to have 'some' sort of order or leader, unless all know each other very well and are used to the idea of 'better' singers getting most of the time.

   (I used to sit and study certain groups and watch the body language of known 'circle hogs' as they leaned forward, waiting for the last note to die away so they could 'take' the next turn with barely a seconds pause.)