Not sure why this is even a question. Sure, it's nice to have low cost units but they get a LOT stronger when you have evolved them. It CAN be useful to keep like 1-2 units un-CC'd at max CR to deal with some situations, but this is not something a new player should worry about and you should just evolve your units once they hit level 50.
They will be stronger and thus able to handle the maps that come later in the game, which the un-evolved units will not be able to.