It’s a refrain I hear (or read) over and over again. Magic-users are too weak, especially those first-level magic-users. Imagine, just one spell a day… then all they do is stand around and be useless.
Except, that’s not how it works. Let’s discuss as an example a party with four characters: Darion the Fighter, Apoqulis the Cleric, Barthal the Thief, and Lucas the Magic-User. They’re on their very first adventure together, investigating the Olde Island Fortress.
Darion has 1d8 hit points, and is armored in chain mail (as he could not afford plate mail for his first adventure). Apoqulis is armored the same way, and has 1d6 hit points. Assuming no special bonuses, with shields in hand they each have AC 16. Barthal is in leather armor, and while he can’t use a shield, let’s assume he has a Dexterity bonus of +1 (he is a Thief, after all), making his AC 14. Lucas is unarmored, AC 11, but of course he’ll be at the back of the party, out of reach of most monsters. Being unarmored also means he has the fastest movement rate, 40′, so he can outrun the other party members. Recall that when a bear is chasing you, you don’t have to outrun the bear, just your comrades…
So they enter the dungeon, and shortly they encounter a group of four goblins. Now, goblins have 1-1 hit dice, so each of them has an average of 3.5 hit points. Rather than deal with the details, let’s just say each has 3 hit points. We’ll be nicer to the adventurers and round their hit points up, so Darion has 5, Apoqulis and Barthal 4 each, and Lucas 3. The goblins do 1d6 damage, or 3.5 points on average, while Darion, Apoqulis, and Barthal average 4.5 points each. Finally, note that goblins have an armor class of 14.
So they fight. Without digging too deeply into the numbers, the adventurers have only a slight advantage over the goblins; the odds are in their favor, but it’s quite likely that the fight will be over in four or so rounds, with at least some of the survivors being injured. Should the adventurers win, remember that Apoqulis does not yet have a healing spell available.
Most merciful GMs will allow a starting magic-user at least one offensive spell. If Lucas has Magic Missile, he can probably kill exactly one goblin. If he has Charm Person, he can take control of a goblin and at least remove him from the fight, if not actually turn him against his comrades. If he has Sleep, it may be all over for the goblins in the first round.
If Lucas casts his one spell, and it’s anything other than Sleep, he probably removes one goblin from the fight. His friends are still fairly likely to be injured; when the fight is over, one way or the other, they will probably need to withdraw from the dungeon to return another day.
And on that other day, Lucas will again have one spell available.
If Lucas has Sleep, well, it’s entirely possible the adventurers will breeze by the goblins without suffering any harm at all. After Barthal trips lightly through the sleeping goblin’s ranks, slaying each in his turn, the adventurers can move on to the next encounter, and it is possible that Lucas will indeed stand around doing nothing while they fight. Of course, the smart thing for Lucas to do is to carry some daggers for throwing, and use them to whittle down the back ranks of their next group of foes; his chance of hitting at first level is equal to all the other characters, after all, so why not?
At higher levels the comparison holds out. For instance, assume the party is now 5th level. Each of the adventurers has a magic weapon, and probably some form of magical protection (armor, ring, etc.) If, in the course of gaining 5 levels, they have found any magic-user-only items such as wands, naturally Lucas has received them. Meanwhile, the number of hits each of the “heavy hitters” can sustain has been multiplied by 5 times, so now they can fight much longer. But still, by the time Lucas has exhausted his own magic (five spells, one of which might blow away a large monster or a group of smaller ones), the adventurers should be ready to call it a day.
If you find, in your game, that it doesn’t play out that way, ask yourself why. Have you allowed the fighter and cleric to receive powerful magic weapons and armor, making them into walking tanks that deal death without suffering a scratch? If you did that, did you also allow the magic-user to receive items of comparable power?
Before changing the rules to make the magic-user more powerful in an attempt to achieve parity with the other classes, you should consider whether the rules are really unbalanced, or whether you have (intentionally or otherwise) stacked the deck against the magic-user in the first place.