Most of the time 2 armor pieces drop, and each boss would drop a different piece. You would get whatever you needed from ~4 runs of a dungeon if you were able to get through the whole thing. Maybe a couple more if you were unlucky. Also, if a dungeon wasn't on full blown out farm mode, it'd take a couple hours to finish the whole thing anyway. Unless you have a full day to waste, running once a day wasn't a big deal.
Ohhhhhh okay I understand now. Makes a lot more sense. Sorry, just never had the privilege to do these on live as I didn't play. But that sounds pretty dang fun!