Yeah, the only Pokemon that gets even the slightest benefit from not immediately mega-evolving is probably Gengar, just for the ability to switch into Ground moves.
Well, there are a few others. MegaMan loves his lightningrod boost before he nerfs your attack stat, and I've won a few double battles against opposing MegaMen because I didn't mega-evolve immediately and soaked up their attacks.
MegaKanga, if you aren't running crunch, can stay normal and put a nice dent into any predicted ghost switch-ins with scappy return.
Don't forget Heracross and Pinsir. Both love getting a quick kill as a non-mega for a Moxie boost before Mega Evolving.