If there aren't two more DPS waiting to hop into that queue as soon as the first ready check fails, then surely you run the risk of them not having joined by the time you reach the boss anyway.

Also do you even get to join alliances in progress like you do for dungeons? I've never seen it happen. Could just be a matter of nobody dropping out of course, but the parties seem to be coded a little differently, like you can't go in undersized without three parties of some description.