Actually, Aggron DEFINITELY need a mega. He sucks. :P
Also, mega evolution may look broken on paper but I got a thought they got a BIG downside to them - they take up precious item slots. Items are a large part of competitive Pokemon, and you're wasting them for a mega stone. Mega stones are essentially the mega Pokemon's greatest enemy, and the biggest limitation. The reason Mega Aggron won't replace Skarmory is because of not being able to have Leftovers. And then there's the type change for some Pokemon who won't be able to benefit from it much. (Gyarados only has Bite for a Dark STAB, Zard's only good dragon move is Dragon Claw, etc)
Also, I beat many passerbys with Mega Mewtwo/Blaziken and all that with my in-game team without a mega. They're very much doable. Granted I'm an ex-competitive battler and I easily predicted them. :x