I find this a commonality as well. Its a difficult exercise to squeeze into a short class period so I think it gets glossed over a lot. It can be tricky to find enough qualified uke who don't mind revving it up some. Also, only one person can be nage at a time...hence the extra time consumption.
depends on how you structure your practice. most folks practice aikido as though gearing toward only one person, nage, improving, instead of both nage and uke(s). uke(s) job is learning to attack well and to take ukemi well. nage learns to deal with the attacks. same thing in randori, ukes portion of learning is how to work in group, i.e. teamwork. how to take ukemi in a group and clear out the space so your other partners can attack nage and not be in the way of each other. ukes learning too, not just nage. when you structure your aikido practice like that, then you don't waste time on folks. and for those who stand on the sideline, not in the current randori, they learn as well by spotting mistakes and opportunities.