I don't think that's necessarily so; a rogue can use Sneak all day long; a wizard might get only use of invisibility all day.
But Rogues do not learn new stuff. They have sneak attack, which is "at will", and it goes up with level (depending on the edition).
Quadratic Wizards means that when they level, their abilities (that is, spells), goes up by level. But they also get *new* abilities.
A 3rd level rogue can sneak, open locks, and backstab. Let's assume that is balanced with a 3rd level wizard, who can cast
invisibility, knock, and Magic Missile[/i]. The rogue can do all the day long, the wizard is balanced because he can do it only a few times per day, but their effects are stronger (invisibility>sneak, knock auto-success, magic missile does less damage but autohits).
So, under that assumption, the 3rd level rogue IS balanced with the third level wizard.
Both go up 6 levels. Now they are 9th level. The rogue can do more damage with sneak, can open harder locks, and is better at stealth. The wizard also is better at sneak (invisibility last longer, now he has sphere of invisibility and improved invisibility as well), he can not only open the door, but Passwall and Dimension Door, he does more damage with his magic missile, and also has AOE fireballs, and even more damaging spells. In addition, he can Summon Monsters, Teleport Away, Dispel Magic, he can create impenetrable Walls of Force, he can Polymorph his friends, can Haste his allies and Slow his enemies, he can Charm Monsters and Fly.
If we work under the assumption that 3rd level rogue is balanced with the 3rd level wizard, then the 9th level rogue is not balanced with the 9th level wizard. They can't be. When you have two functions, and one of them is linear and the other is quadratic, they intersect only in one point. In our example, 1st level wizard is worse than 1st level rogue (he can't be invisible, he can't open doors, and his magic missile is pitiful). They are on par on level 3 (as per our assumption. Maybe you think they are on par at level 4, or 5, or whatever other level, but that's irrelevant to the point). Beyond that point of balance, the rogue keeps advancing linearly, while the wizard keeps advancing quadratically. So, past the intersection point, the two functions never cross again.