They're more for strategy than anything. Though they can help you level up a specific character quickly if you use them liberally.

Basically, they aren't the demi-gods they were in the previous installments. But they are useful for exploiting elemental weakness'. A few of them are still pretty useful. Chaos, Ultima, and Zodiark are all still useful by the end.

The main problem with them, is they have no defense, so if you don't watch it, they get killed quickly before they can do anything useful. The end game summons also have weird conditions to activate their powers. They're not as bad as people say you just have to watch them carefully. You can't just summon them and watch them wave a path of destruction, they can do it, you just have to watch their health and you have to give them a few buffs (like Bubble).

I personally like how well balanced they are compared to previous games in the series.