haha i never thought of eidolon`s that way :P
i find eidolon's useful at the end of an battle. im not really a fan of interactive summon either.
in the beginning of an (boss)battle i always use .. euuh.. jeez its been a while since i played XIII ... the role than can put enchants on yr characters like Haste and Shield.
So i pump the characters up quickly with 2 of those.

but when u use an eidolon all those stats are gone on the other 2 characters