Mega Evolutions are a better solution to strong Pokemon that aren't quite making the cut but have high stat totals than evolutions; most non-legendary Pokemon won't go over 540 BST, and evolutions that involve stat redistribution are a bit messy; compare 2 Pokemon with the same BST, one with a regular evo and the other with a mega:

http://veekun.com/dex/gadgets/compare_pokemon?pokemon=Aerodactyl&pokemon=Porygon2&pokemon=Mega+Aerodactyl&pokemon=Porygon-Z

Had Porygon-Z been Mega Porygon2, it could have retained its bulk, gotten the boosts to Sp Atk and Speed, and still have 40 points to distribute. Instead, we got a slow nuke whose preevolution is better due to bulk, while we could have gotten a 85/80/100/135/105/110 monster with Adaptability, able to come in to get a Download boost (or fuck something over with Trace).

We also have cases like this one:

http://veekun.com/dex/gadgets/compare_pokemon?pokemon=Dusclops&pokemon=Banette&pokemon=Dusknoir&pokemon=Mega+Banette

Dusclops and Banette were both in a grey area; both a mega or an evo would be quite fitting (although a Mega would only boost them to Arcanine-level BST). The problem is, Dusknoir got 70 points upon evolution, and a worse spread wouldn't be possible. 20 points to speed on something that slow actually hurt it, seeing it no longer outspeeds Snorlax or Slowbro in Trick Room. The extra attack is nice, but not that needed; Seismic Toss and Pain Split exist. And the negligible boosts to the defenses of a defensive Pokemon cannot be understood. Mega Banette could work as a non-mega, that kind of stat increase is what those kind of evolutions had to be.