Here's my problem (Which might have already been state, who knows):

Let's say you are just some rank 50 dude looking for a rank 30 piece of equipment just to use while leveling another class. You need to drag three other people along just to get it, and as a result, powering them through the dungeon. If you've ever played any other MMOs with uncapped dungeons like this one, lower level people will always be up for a high level dude mowing everything down. The one thing that keeps this system in check, however, is the fact that the high level dude would have to go out of his way to bring some people along, which most don't care to do. Well, that and most MMOs drop bind on pick up equipment anyways, and don't allow for multi-class characters. Unless that high level dude is a friend of yours, he most likely won't go out of his way to find lowbies to tag along. He'd much rather just go through it on his own time, when he wants to.

If there were no restrictions on minimum party size, I'd say keep the instances uncapped. But since they do have a minimum requirement, you might as well cap them. This is just some weird "middle of the road" system that I think should be changed in some way. I'd prefer an unrestricted party count over a level cap, to be honest. But I'd be fine with either.