Personally I dislike the Trinity system as it is right now, but I also dont think every class should be "Self-sufficient" as this is an MMO, not a single player game.
Ive discussed this sooo many times with my husband I've lost count. I think the trinity system should be ...expanded.
Some (maybe not all) classes could be hybridized to fill 2 roles in the DF system. For example...
A Tanks could also be a DPS if given some abilities that would make their DPS on par with other DPS.. (Obviously while a tank is employing a DPS role, their tanking ability should be limited. So if a tank was filling the DPS role, that tanks' ability to tank should be affected in some way.)
We can already see this to an extent with some classes (ie: SCH/WHM with Cleric stance, and WAR with its stances) but build upon it so it can actually choose to go into DF as a DPS OR as Tank or whatever. Or SCH could become a DPS but give their fairy some magical attack capability. There are myriad ways other classes that could fill other roles if given the abilities to do so.
OR (the better method in my opinion)
Make a new addition to the trinity system. Add in SUPPORT specific roles as an option to the duty finder, and bring out some classes that fill SUPPORT only roles. For example Corsair/Bard/RDM in FFXI. Each of these classes brought some crowd control or support abilities to the party, but their DPS/Healing capability was much lower than that of other classes to reflect the fact that they could boost or aid the party in other ways. For example, FFXI Bard had slow and haste songs, regen songs, a poison debuff song for enemies, etc. Corsair had similar abilities and the ability to boost exp for short periods of time. RDM could fill many roles by being able to do dmg AND heal but to a lesser extent, they also had myriad debuff abilities such as Haste, Slow, Poisons, Stuns, etc. They were an all rounded class that could aid the party in many ways.