As people say the system currently doesn't try to balance the 2 dps. It would also make queue potentially longer for no reason assuming your dps catégorie is currently highly played.

Also, assuming no shift into the player base, it would shift the long waiting queue from dps to tank/heal. According to data before 4.0, something like ~45% of the player base are heal/tank. Which ends up being a a good balanced since half the group are heal tank and the other half dps. If you'd make light party 5 man, you'd need 20%heal,20%tank and 60% dps which isn't the current pool.
Consider how long the queue would be if we went from ~5% missing to ~15%.

In HW tank complained about this for raids since those group already have 3 dps.