The queue time for Leveling (stop calling it low level, 7 of those instances are 51+) as DPS is usually not longer than 6 minutes for me. So there is already enough tanks and healers queuing for it. There is no reason to "get more of them in" by allowing queuing as a premade, especially since it's not guaranteed to open more groups for DPS, it could easily have the exact opposite effect since 1 or 2 DPS could queue with a tank friend who could otherwise be used for random players in progression dungeons.