We already have play-styles that care about allies. Worship Beogh, Fed, or Oka. They're the guys who expect you to keep allies alive and train them up, and who will give you arbitrary punishments for mistreating your creatures.
Summons though... as they are now, you're magically wresting beings from other planes of existence or shaping them yourself from the firmament of the world, and binding them to your will. They can't chose to resist the calling, or being created (if the spell fails, that's you're incompetence), but they
can resist your will (hostile summons). Summons are not currently creatures you form contracts with and summon / banish again and again a la Tales of Phantasia or Final Fantasy.
We could change this, but it doesn't seem flavorful it me. I mean, if wer're forming contracts, you really need sidequests where you meet / prove yourself to the creature to earn it's support- calling them up out of the blue using a spellbook doesn't sound right anymore (you'd need a hall of summons branch? Contract portal vaults? Dunno). Also- what about the necro/summoner overlap? It really doesn't make sense for anything necromantic to care about preserving, caring for, or using your allies as anything but disposable unnatural ammunition.
Also- it seems to me it would make summoners harder, not better- you lose the ability to use disposable meatshileds as an escape option, and you run into the HOPr problem where if you lose a heavily invested high end ally in the late game, you're doomed. This would also discourage other classes from picking up low level summons to use as escape spells, since they'd become useless fairly quickly (I know there's talk of nerfing butterflies, but this seems a little extreme to me). The only real advantage I see is if this system were imposed on monster summons as well- it could be used to nerf summon spam and chain summoning.
That's my two cents for the status quo!
Edit: dang, ninja-ed by dpeg.