I foresee a problem with necessity vs. optional-party-member when it comes to preformed parties completing duties.

With the current trinity, (in theory) any job can be brought to fulfill each of the roles. Bringing along a SMN and a WAR should not be any different than bringing a MNK and a PLD. The fact that you bring one DPS and one tank is sufficient for those two slots.

However, if we introduce support roles, there will inevitably become a struggle between needing a support role and optionally bringing a support role. Duties would either have to be balanced to allow both support and DPS, which would be nigh impossible. As clearly an extra DPS would bring in more damage output to kill things faster.
Or the duties would strictly cater to support. Meaning, you must queue with support. And nobody wants that.