The question is - how much do you want to put the spotlight on combat? OD&D didn't as much, by default (although a number of people did play it that way).
Ummm . . . what?
Which OD&D are you talking about?
The one that wasn't a set of converted and kludged miniature wargame rules?
The one that didn't have to wait for revised editions to have more than a vague acknowledgment that towns even existed?
The one where people other than thieves had actual skills in the core rules?
By default, the game was about
dungeons (plus those dragons of course), where you engaged in combat (plus some trap survival).
Strongholds, sometimes towns, existed only to provide a safe place to recover hit points between extended bouts of combat in said dungeons.
Towns and cities existed only to provide justification for trading lousy loot you got from combat in dungeons for whatever better loot the DM allowed you to have, plus to recruit henchmen to do more combat in dungeons.
"Role-playing" was bragging about how cool your character was in combat compared to everyone else's character.
Background and setting were ad hoc things kludged and improvised at the last minute, focused on getting players back to combat in dungeons.
The spotlight didn't have to be on combat as combat was the only thing allowed on stage with everything else kept under lock in the prop closest.
You can have spikes of power, but casters need to be doing something useful almost every round. That can be "building up power for an awesome spell", but only if that spell actually gets to go off.
It can also be firing a crossbow. (Or throwing darts back in the old days.)
Or tossing flaming oil. (Alchemists' fire since the new millennium began.)
Or dumping the loot on the mule and opening the door for a quick getaway before the fighters get slaughtered. (Which is why I like
servant horde,
floating disc, and several
pearls of power myself.)
Not to mention the more fundamental debate between glass cannon blasting and being divine rank -1 battlefield control.
Have to define "useful" before seeing if the spellcaster is "useful".
Also, I find it a little annoying that this thread has gone from "we should give the martial classes some bad-ass abilities" to "meh, just nerf casters". I mean yes, 3.x casters reach heights of power that snap the entire Monster Manual in half, so they can go a lot lower. But balancing to the (crappy) level that martial characters are at there is no better.
IF, Tier 3.5 is the intended power level;
THEN, by default you have to both buff the Tier 4-6s and nerf the tier 1-3s to achieve "balance";
AND, by definition that is neither as sucky as martials nor as broken as the casters.
Even without going that far and just keeping a distinct differential between martials and casters that is "balanced" by character level and demographic distribution requires both buffing of the weak and nerfing of the mighty without making either cross the line.