Elves can't build catapults. No fireball bombards equals no city capture for them. Maybe earth... but I'll stay away from that, as I hate Summons.
Fireball is 4 fire damage and inflicts collateral, which is why they're good to begin with. M2 summons are often 3-5 str ( why are they ever 3? ugh, looking at you lightning elemental) and don't cause collateral, making them generally quite useless when you consider it took dozens of turns to rank up your adept to Summoner and it has to repel or overwhelm a stack of Champions it took your enemy 3 turns each to build.
You gave every reason why I would like to see it moved to something else.
Elves could still get it another way, like through earth. At least there would be some reason to use SOMETHING else besides fireballs, for everyone, not just the elves. It's like you said "Golly, elves wouldn't get every benefit of the entirely unthematic spell that is really the only one worth using at that level anyways!" They could just use whatever it was switched to for sieges, and preferably lose fire mana from their palace. Just my opinion, but there is just too much fire in the game. The bad guys use it, the good guys use it, the Elves use it, etc etc etc.
And if the palace was changed, they could even still get the collateral damage from fireball by building a node, just like everyone else. I am really not sure what your point is in regards to the OP.
I find summoners to be extremely effective, myself. Sand lions and the death 2 summons work great for me. However I do think the 3 str ones could be bumped up a bit, or at least given more affinity.
For the record, I hardly ever bother bombarding a city one way or the other.. summons and mass damage/disease spells like Air 3, Contagion, and Ring of Flames do the job just fine without the bother of lugging around a catapult. And yes, I only play on Emporer and above.