You say that we can use several spells to generate XP out of nowhere, perpetuum mobile? It contradicts laws of thermodynamics, and while I know we're talking about magic in fantasy setting, I'd find it strange to not apply it onto well-developed game system.
If, by some bizarre way, you manage to get an enemy wizard to keep throwing summons at you without killing him or getting killed in the process, which is inevitably halted at some point, as there's a limit to spells per day after all (after which you can just kill him), making this assumption pretty pointless, then I guess so. Note, however, 2 things: 1) the mage must be an enemy, party summon friendlies exploits are excluded. 2) the XP isn't generated out of nowhere, you're just bringing along a creature that was somewhere else at the point previous to the casting. Teleporting/plane shifting to its location instead and killing it would accomplish the same effect.
Taking your extreme example to the extreme, taking an enemy wizard prisoner and giving him a spellbook of only summoning spells and engaging him each day under monitored and regulated conditions by the party wouldn't yield XP because the DM could rule that there is no actual challenge there. Blatant exploits aren't part of my argument.
In that case, you get XP for completing the test, as in Quest experience. The fact that it involves fireballs is irrelevant. You can get dozens of thousands of XP just by answering a riddle, after all.
I'm talking only about the concept, not game balance.
I guess the discussion continues, then.