While I largely agree with you, Peter, it's not quite that simple because re-running quests (or helping with catchup) still gives XP. Not as much as that first run, but still XP. The reason Az had to jump around 3 toons is because one of them was not only holding a level to stay with the group, but was actually extremely close to the next level from running catchup with Reywindd.
Our trio ran different quests in 3 Barrel than JM's quartet a couple weeks ago. If everyone is going to complete, that entails a fair bit of repeats and/or PUGs to bring everyone back together. Those repeats are still giving XP, and unfortunately the people that seem to have the most "overlap" are the ones that, for various reasons, need the extra XP the least. We're also limited a bit by function--we've only got a couple healers and a couple trap monkeys. Those toons will be needed in all runs, so it will be hard to keep them from racking up too much XP. The healer side might not be too bad since Wouldii's a 2nd lifer with no tome and no pots and your cleric is a 3rd lifer, but we're really up against it with trap monkeys.
I'm probably trying too hard to make it all work (it's my nature, what can I say). I think the problem will ease up some as we progress since levels will come slower and we won't be trying to pack as much content into the available space. Until then, I'll keep trying to make it work for us without getting overly dictatorial.