Wow. When I DM, the only way that I ever get two or more battles into a single day is either to make them trivial or to use ambushes. Players use teleport, hidden lodge, rope trick and other stunts to race back to safety after hitting hard. The result is that I always see a full caster nova effect where they unleash limited abilities (sudden quicken, divine metamagic quicken, pinnacle spells) to annihilate an opponent and then flee.
When we went through RttToEE (3.XEd, and I was a player, not DM), we had a stretch in which we went through 6 combat encounters without an in-game break. (It covered 3 sessions.)
The single-classed mage still had a spell or 2 left at the end of it. He had spells left because instead of nova-ing, he'd cast a couple of spells, then reach for a weapon.
The main healer was tapped, but there were other (minor) curatives available. No front-line warrior had more than 20% of their HP.
By the time that stretch was over, my Ftr/Rgr/Diviner/Spellsword was getting ready to move up to the front...and he'd been there before.
We didn't rest before then because we didn't have the opportunity to. Though we had made occasional returns to our base of ops to restock & reload, retreat was not an option at that point.
And that's typical of the way all of the DMs in our group run things...even before 3Ed.
If the wizard doesn't have very many spells, he runs out of them way before he runs out of rounds of combat. Then he's got nothing he can do well. He sucks at using a crossbow, and he probably won't invest resources in getting better at it because those resources will be wasted when he reaches higher levels and encounters problem 2.
Well, in 3.X, that depends on his build- a ranged touch spell afficionado will be nearly as good with his crossbow as with his spells, since he'd probably take feats that serve both pretty well.
But even before then, I never had a problem with this.
My very first D&D game back in '77, my Fighter and another player's Mage were the last 2 surviving PCs when we encountered a Purple Worm. He launched his last spell- Magic Missile- then fought the rest of the way with his staff. For a while, he was outdoing my 2hd swd equipped fighter.
No, he never was intended to be a melee combatant. But the fact that he waded in nearly made the difference.
This could be fixed by giving wizards something worthwhile to do when not casting spells. And then making that something worthwhile stay worthwhile for their entire careers. You'd also need to reduce the number of spells per day drastically.
The Reserve feats were an elegant way of handling this issue: they give the spellcaster something to do instead of casting spells, something at least as good as using a weapon, and its still "magical"- hopefully satisfying those who lament resorting to using crossbows as "unwizardly."
And ditched in 4Ed, near as I can tell.
Even without those, the creation/use of alchemical items, the use of KS skills and so forth are still constructive uses of non-combat time for spellcasters.
YMMV, of course.
From a combat perspective, everyone had MM, Sleep, Web, Invisibility, Haste (etc). In early editions, the wizard did not have that many slots, thus these Power Spells were critical.
Even into 3.x, these spells were still the best in class at their levels and you saw very similiar spell selection by players (DMs could use different combos as the Sorcerer/Wizard had a life expectancy of 1 encounter).
I realize you're speaking in generalities, but...that was never my spellcaster. I may have had a PC with one or 2 of the "power" spells, but never anywhere near half of them...and NEVER Magic Missile.