I'll put it this way.
Dagger's summons are useful when you first get them, but by the end of the game, they are worthless if you did the Chocobo Hot and Cold sidequest, killed a lot of dragons, ate a lot of frogs, and/or stole a lot.
Eiko's summonns are far more useful. Carbuncle's Ruby Light casts Reflect on the entire party, and if it has the full animation, Protect. Reflect on the whole party is useful if you have Vivi, and, to increase the effect, have Reflectx2 equipped on him. Phoenix is a useful reviving spell if several party members are KO'd, plus damage is useful. It has a chance to appear when all party members are KO'd, reviving them. Very useful. Fenrir isn't that good, but switch around his Wind attack and his Earth attack to match the weaknesses of your enemies, and he becomes somewhat useful. Madeen is an excellent summon when you first get it, and is a good summon even at the end of the game. Plus, the Ribbon you need to learn Madeen has many other useful abilities to learn from it, and it gives a good stat boost.