A while back, I had this same discussion with my FC and someone brought up a very good point. There are a percentage of tanks and healers that play those classes because of the shorter queue times they get for doing it. Now, if you shift the party make-up to include more dps, then the dps get shorter queue times while the tanks and healers get a longer queue. How long will it be before those people playing tank or healer classes shift to dps classes and put us into the exact same situation that we have now?

From a technical standpoint, increasing the number of dps in a party would require all of the current dungeons to be redesigned. Additionally, many healers use the time when they aren't healing to help dps. If they suddenly have another dps player's health to keep track of and keep alive, then their play style would likely see a large shift to more pure healing.

In the end, adding an extra dps player to light dungeon parties would only help the dps players have shorter queues while making life worse for the tanks and healers. If it is difficult to get people to play a tank or healer now, do you think they will stick around when one of the main benefits to playing one is taken away or their entire style of play has to shift? I main red mage now, but I do have paladin and white mage leveled and decently geared for daily dungeon runs if I want faster queue time or a change of pace. Without those faster queue times, I would just stick with my red mage.