It's a good thing Metamagic is rare, since this opens a nasty can of worms.
You wouldn't want to meet a Blade Barrier that, unknown to you, happens to be Maximized. And there are some people paranoid enough to put such a thing permanently between the top of the rubbish chute and the otyughs below.
Why wouldn't an Artificer pay a little more to have, say, that +1d6 fire attack he's adding on to this fine masterwork Greatsword.. "yes, let's get that maximized, +6 damage is so much nicer." "I say, brilliant, good show old chap!"
*Balance alarm goes off* Does this bother anyone else? Those Flaming Acidic Holy Shock modifiers on the theoretical masterwork Greatsword coming at you aren't just potential D6 any more.. that's a full +6 fire, +6 acid, and +6 electricity damage coming at you, and +12 more to Evil folks. Mmmm, taste the +30 damage goodness.
Yeah. That's a lot.
I'm seeing a slippery slope here - best to keep Maximize away from the weapon spell effects, and let the weaponsmakers rest better at night.
Don't teach Metamagic feats to disreputable ruffians!