Oh true about the reverse Elves = sorcerors you can rivet another class onto - in theory. The issue in practice is the nature of fireball slinging and how it revolves around sweet spots. (Except for bloodmage, who's does better as a dwarf or halfling than anything else tbh).
Thing is, what you really want with your big mana fireball chucker is sweet spots, >10 mana thresholds which give additional fireballs. An elf needs to work for theirs, a sorceror... doesn't. Elven boots are impossibly easy to locker early, you locker those, you're at 18 mana from the moment you step in. If you find mystera you're at 20 easily AND with mystic balance that's your new sweet spot. For doing nothing, really. It's easy even with a prepped mystera, and you don't even have to find a single natural mana powerup. Or if you want to shave off 25 Mystera Piety you can prep the subdungeon and have one right next to the entrance. And there's other ways to go about this, as has been mentioned.
And then the difference really shows - an elf CAN hit this sweet spot, but he's 5 mana behind so he has to convert stuff and has fewer initial mana to spam for piety so he ends up wasting more blackspace on it (mystera piety generation costs blackspace when you think about it). So he can get to 20 mana + mystic balance, and the sorcerer kinda sorta starts out there or at least takes way less effort to get there. After that, if you want another fireball, and elf has to come up with not one CP pop but 5. Less than that is mostly useless, so a relevant CP ammount to get something out of an elf isn't 75 anymore it's 375 - AND he's allready had to burn CP to get to 20.
Now since refreshment gives 50% of your mana pool, 20 + Mystic balance IS the sweetest sweet spot from the cost/effect standpoint since you get 2 more fireballs per glyph. If halves are rounded down (and I suppose they are) with 25 you don't get anything (you get 2 mana, so you need 3 glyphs to get one more fireball out of it) and you need 30 mana to get 3 fireballs. You can turn your mana potions into 50% with the cauldron so the same math applies. So if you wanna really get somewhere with an elf after mystic balance and 20 mana you need... 750 more CP, and it allready took you CP to get to 20. If you're refilling via refreshment, you'll have a hard time doing without burning all your refreshment fuel (glyphs) so you'll reeeely have to overprep for CP.
So going there isn't really worth the hassle, IMO, unless you're dead set on doing it. If you can have the sweet sweet 4 fireballs 20 mana setup with no CP involved and use all the CP for any kind of refill (dings or potions), you'll beat the elf for efficency every time. Plus, I'm a big crystal ball abuser (it used to have a non scaling cost, it was ridiculous) and I didn't really need elves - CB and schadenfreude for 4 fireballs and way less effort was enough for me to do any vicious and most VT runs.
Yeah, it kinda goes both ways - why use the Sorcerer when I can just use the elf, and if you approach it from that direction it the sorceror looks rather bland (+5 mana? So he's a weak elf then? What do his other abilities do?), but when you get down to bussiness and execution the elf turns out to be an "inefficent sorcerer". Or at least has for me.
As for the places other than fireball chuchking (or specific elf-friendly classes) where an elf might apply and you obviously cant pick a sorcerer, here's my experience:
Warlord - there's a lot of competition. Find/prep EM, and the size of your mana pool ceases to be relevant. This is true for most any spellcaster but Warlords who prep EM can go ORC and really disregard mana pool building completely. Go mystera or JJ and a gnome will serve you better. Go cauldron + EM + Halfling, find cydstepp - you don't even have to be a warlord. Cydstepp has even more spread out sweet spots than burndayraz so they've been even more about the refills than the mana pool size for me. If you can hit the 2nd threshold, only an elf can get to the third reliably, but the CP cost is intense.
Assassin - honestly, I haven't played too many spellcasting Assassins. I can see how elves can work there, and you obviously can't pick sorcerer and assassin any other way. But if you could easily hit the sweet spots without being an elf (and you obviously can what with all the goblin assassins running around) you'd never even consider elves.
Fighter - here I do pick elves, but mainly because I tend to go for TT for obvious hilariy, and that cuts me off from mystera, which also means no mystic balance. And it's not like you can't go Goblin or Dwarf and go nuts, or even Human/Orc + TT and take stuff out with potions.
Oh, and elves don't have any sinergy with B2P. Any dwarf or halfling who runs into it can go nuts with spellcasting (see Astral's halfling transmuter sillyness for blowing it out of the water).
EDIT: So sure, if the PQI sends you to a dungeon best approached with a big mana engine and fixes your class for you to be something with no enablers for big mana, you'll likely go elf (but there's tons of ways to go about it, really, once you unlock everything). But as long as your class isn't pre determined, and you think about it with a bit of logic and experience and don't want to specifically challenge yourself, "elf guard++" sorcerer probably is the superior choice. Again, at least it was for me...