Its more complicated. A DRGs contribution, for example, is vastly higher if there is a SAM in the group than if there was a Summoner, because of the way his buffs and positioning usually works out. Certain combinations work better than others, and thus certain setups will vastly outperform others. SAM's value is that he slots in more easily than the old MNK into more different setups, less picky about his support, and helping out tanks (thus freeing up choice there and not requiring a Ninja) with slashing debuff.

Mantra, as almost all defensive support, isnt all that good because healing (so far) is easy and not an issue, same goes for mitigating damage. Its only about DPS, and there lies the problem.