Would it be a disadvantage NOT to evolve certain Pokémon?

[aegis]

like everyone else is saying, it only matters if you play competitively. but, i never care about that - i just keep/use the pokemon i like and i'm happier that way. you should play the game in the way you want to, as well.

 

[Silversea]

You can mess around and be competitive. I taught my eevee Trump card cause I like the move visual personally, even though its mostly hopeless in competition. But I still stand up in battles fine ^^.

 

[Zeiro]

Ash never evolved pikachu...

In one of the earlier anime episodes, he almost did. He had a thunder stone and everything. Both him and Pikachu chose not to use it.