I have thought that a "zipper merge" (let one car in ahead of you) convention would help congested roundabouts but that isn't how we're taught to use them.
I'm not sure how that would work. Now the person entering the rotary has to trust that another driver isn't going to cut them off. I can tell you that merges right after a traffic light it is absolutely routine for a car (or probably more likely a pickup) not to let you alternate. I don't even bother with the FU horn any longer.