I spent a good three minutes typing up a comment here and as usual, Blogger eats my comment. Arg.
I was gonna say:
I like the one-and-only summoned being idea. I’d connect it to the infernal/celestial being’s true name. You learn the spell, you learn one true name of a creature, and if you want to summon a different one, you need to find another name to use. This also would make wizards very reluctant to sell a copy of their spell, as now someone else might be “using” their little extraplanar buddy or “tying up the line” when they want help! Maybe a swindler will sell you a spell with an invalid or wrong name. Maybe a demon named “Insert name here” is terribly overworked and tries to get the party to collect all known copies of the spell with his name in it, for a reward or whatever. And scrolls with a few true names would be desirable treasures. (Sure you know “Summon,” but all that doofus you learned it from knew was the name of a celestial muskox and you’d really like to be able to summon up a celestial peacock or whatever.) Then again a really old scroll might only have the names of now-dead-as-in-killed-on-their-home plane-and-really-really-dead creatures. You should run with this idea…
I like it; has a nice Earthsea feel to it.