>>58483892
Well, yeah, there is a tutorial fight where you get a temporary lucario and you will need to mega-evolve to finish it.
Also some side missions where you need to fight with a specific mega-evolved pokemon.
But now that I think about it none of the main 'story' missions require mega-evolution.
And even though its infinitely more efficient to mega-evolve. you can beat the bosses with plus moves instead.
But seriously just use a mega, there has to be at least one you like.