You kinda don't wanna have 4 Pokemon that can mega evolve on the same team. Too many Pokemon that can go mega need to go mega so if they can't they're kinda screwed. In VGC you can do 2 at most. Blaziken at the very least functions fine without its mega stone. Salamence you can probably get away with not mega evolving, I'm pretty sure it's still viable in the base form in VGC. Metagross isn't completely awful in non-mega form but very much prefers being Mega, while Manectric is straight up **** without it's mega stone.
tl;dr: give Manectric and Metagross their mega stones, they need it the most