Marowak's hold item brings it to slightly above Medicham's Attack when said Poke doesn't have a hold item. Mawile hits 'bout as hard as a Medicham with an Orb, has better typing, and will be noticeably bulkier. It's essentially a 130 Base Atk Poke with a Choice Band, minus bein' move-locked. And it's a Steel/Fairy type, quite possibly the ultimate anti-Dragon typing. If you think Mawile isn't gonna be OU, top threat or not, I call bullshit. We'd need a ridiculous power creep for a Poke like MegaMawile t' not be awesome.
'Course, your point is valid for other Mega Pokes. If stat boosts for the offensively-oriented Mega evolutions ain't notable, it's wholly possible a Band/Orb/somethin' for a more specific strategy might be more viable than a Mega stone. 'Specially when considerin' the restriction against Mega Stones, whatever said restriction is. For example, if Lucario only got it's Atk kicked up t' Base 130, I think people would still use Banded Terrakion more for a CC-spamming Fighting-type.
This is especially true for Mewtwo. Specs makes it's Sp.A retarded, there's no way MegaMewtwo's Sp.A will be able t' rival that absurd power. MegaMewtwo would never be used for raw power, which is kind'a odd considerin' it's a mega Mewtwo. 'Less the stat gains are very notable, offensively-oriented Mega evolutions are gonna be used mainly as stat-up sweepers. In Singles. That's my prediction. Though, I ain't got a guess how it might go down in Doubles.
... On that note, I fear for MegaBlaziken's spot in OU.