Only FF game I've played is FFX, and I found that Summoner was just plain ALWAYS the best class to have in your party. Granted in FFX you can gain the abilities from any other class through the sphere grid. But Summoning is a unique skill. Yuna started out as the party healer (which is damn useful), and then moved into Black Mage territory to become a fearsome magic-user, then moved into thief territory for some thieving prowess and dodging ability.
Is there any other character who could summon an Aeon and deal over 300,000 damage in one turn (Anima)? Or even possibly 15x 99,999 damage (Magus Sisters)? Was there any other character who could insta-kill anything (Yojimbo)? I think not. Most of the other characters had their damage max out at 9,999 until you (annoyingly) found all the parts to their ultima weapons.
Even when you did max out their weapons, your damage rarely went above 30,000.
She just got broken once I unlocked Holy, Flare and Doublecast.