No. An exploit is abusing something overlooked in the system for benefit. Have people done groups of nothing but mostly healers in previous 24 man stuff? Yes.
Why are people going as healer in the newest 24 man stuff? Because of survive-ability. More people have more more MP to recover everyone's HP, making it harder for the enemy to KO you.
It's not an 'exploit' as you're calling it, it's called a strategy. Are you booty bothered that someone's strategy managed to kill your team and pull out a win?
Also I'd like to add.....if one side has a better strategy against the other side, it does NOT NEED TO FLIP.