Is there a benefit to using unevolved Pokémon

In PRO, unevolved Pokemon require less EXP to level up. Raising unevolved Pokemon is thus much faster than raising evolved Pokemon, since the EXP curve is more steep as well.

The caveat is that if you accidentally forget to evolve your Pokemon, they will not be able to evolve. This applies specifically to those Pokemon which evolve by level up. You can ask the staff to help force evolve the Pokemon which evolve by level up alone due to a game glitch, but specific level up (such as levelling up in a specific location) cannot be forced.

To be on the safe side, I would recommend allowing your Pokemon to evolve when they hit level 97 (Level 98 is the last chance you have if it is a 3-stage Pokemon).

Last note, I wouldn't recommend doing that until you completed the main story line, when you unlocked all the areas where you can grind. Otherwise it can be a struggle to progress in the game.