Heres what I'm understanding. Summoner has really good single target DPS but their issue is mana because of it being so small, their damage fluctuates a lot throughout a fight and just isnt as reliable as another DPS. The utility they have is nice, but something like res does more damage than good because of their small mana pool making them deal less damage until thye use several Energy Drains and Aetherflows. A Black Mage doesnt have a res utility, but their DPS is reliable and consistent throughout the fight.

I've heard people say movement hurts Black Mage dps and favors Summoner. I have also hear movement isnt that big of a deal as people make it out to be.