The fact of the matter is that quite frankly, there is almost no benefit to queue for dungeons you've already cleared.

Based on my own impressions, I would imagine that within a month or two, queuing for a sub-40 dungeon will likely take a very long time simply because there is no incentive for people to run these dungeons, and especially not if they are in a healing or tanking role where knowing how to play is a component. There already seems to be a serious lack of healers queued for lower level dungeons.

Even exp wise, once you're high enough to be synched with that dungeon, you still earn more from completing a handful of fates.