This would in fact hurt queue times quite a bit as others have said. If they added an official support role, BRD and MCH would almost certainly have their damage retuned (downward) while their support abilities would be increased, and the content development focus would undoubtedly change as well, no longer taking the support class's DPS into account the way it is now.

This would likely cause a number of BRDs and MCHs to switch over to other DPS jobs, increasing the number of DPS players in the DF pool and reducing the number of support players (as others have noted, support roles are generally not much more popular, or even less popular, than tanks and healers).

While I'm not a fan of the Trinity generally (I greatly preferred the setup in FFXI myself, where classes were designed with various strengths but could be used creatively outside of their intended functionality), it is the simplest way to ensure that all classes are relevant in content. The more class design deviates from it, the more likely you are to see classes sitting out or completely ignored.