I wouldn't allow it in my game, but that's not what you asked.
As others have mentioned, it
is going to make those classes more powerful than any other for that level. If you're playing a d20 Modern game, giving a third-level character Magic Missile at level 3 (2d4+2 damage) is a bit like giving them several shots with a small pistol that can be carried anywhere, passes all metal detectors, can't be confiscated or found by frisking, and leaves no forensic evidence to implicate the caster.
The speed with which such spells start threatening massive damage is scary, and unless you're ALSO giving out save-boosting items, your game is going to get a lot more lethal, and probably not in the fun way you anticipate.
One decent (I think) compromise is the Urban Arcana PrC, the Archmage. A mage can get into that class after a few levels, and one of its abilities is "Count all character levels for purposes of determining your caster level". The key is that you don't get that ability until you get the archmage level, which you can't do until you reach at least 9th level as a character (Spellcraft 12 ranks). So at 9th level, you could be a Smart3/Mage6, and your fireball does 6d6. But at 10th level, you're a Smart3/Mage6/Archmage1, and your fireball does 10d6 because as an archmage, you count all character levels for determining caster level.
Getting that bump at 10th level doesn't unbalance the game as much, because at 10th level, people have better saves and more hit points.