This is also because, especially at level-0, funnel-time, i think this would make a huge difference between PCs own different sets of abilities. The way it is now, you get a 9, you get a 12, there's no difference. You still get the same 50% chance of succeeding on a DC10 check. The other way, you would get only a 45% chance with a score of 9, or a 60% if you've got 12.
I don't know, maybe it's just me, but I get frustrated when (being used to 3.x D&D) I get lots of 12 and then remember they're "just as good as" any other 9, 10 or 11.
I don't think this change would modify the balance of the game, since attacks, spell checks, damage, AC, initiative and Saves would still be influenced by bonuses. But it would make each character more unique.