In their defense, R eidolons can make a decent main if their effect is worthwhile. Their hp/atk doesn't significantly weight against the total, so you can just fill the rest of the slots with the highest stats to keep overall hp/atk up. They can more than make up the difference with an effect that synergizes well with the whole team. And if the main's summon ability is worthless, then don't summon it; the main's summon ability isn't more significant than the rest of the pool, so just make use of the ones you would want. R eidolons' summon quicker though, so you could just tap them for quick jabs while waiting for the heavier hitters to become available.