Berserkers can be very dangerous when used correctly. They act like Blind, but cannot be dispelled with area-of-effect or mass damage spells. So, if the opponent doesn't have Dispel, you can effectively remove several units from the battle for a long time (unlike Blind, these units can also be attacked without penalty!).
You can also do many other things, such as: take high-level resource nodes with a small group of archers, a support unit, and one Berserker.

