I hadn't given this much thought since I never use the item.. but what are the probabilities for each summon that it would select?
The only valid summons AFAIK are the following:
Chocobo (random single target)
Mist Dragon
Titan
Ifrit
Shiva
Ramuh
Leviathan
Odin
Sylph (random single target)
Bahamut
The ones that are not applicable:
Asura (since the summon defaults to targeting the enemy, it's a tad impractical to cast... plus the extra randomness of her spell selection is the other problem).
All monster summons...
Goblin/Imp (it sucks, so who cares?)
Bomb
Mage/Mindflayer
Cockatrice