The only time I've been in a paddling group (that wasn't a class) that used scheduled breaks, the breaks concept was a good one because it means you're less likely to hold off peeing until you're ready to burst...with nowhere to stop. Also good for the body's metabolism to feed it regularly instead of waiting till people risk getting spacey or irritable.
Only complaint I had was that the breaks came too often. Ours were every 30 to 45 minutes! The "leader" needed it for stretching. But that's part of paddling with a group, I guess; things aren't exactly like I might want them to be.
Regular breaks would also be a good time to check weather if it is changing for the worse.
Those are physical aids to keeping a group happy and together. The more complicated part is the psychological side: personalities. That's the part I am a bit gunshy about. Would be interesting to read experienced leader/guide comments about THAT.