First: the movespeed is not reduced, but improved- read 0.8 as 0.8 of normal move speed
Second: this system does not punish you for killing to much- it, in fact, forces you to kill a lot to get proper berserk going on. However, to prevent you from becoming invincible, there are penalties. Also, as I said before, it adds tactics to the AoB- you need to time yourself properly- Game Of Tactical Bloodshed, as somebody once put it.
Third: while chainsaw + brute 2 + berserk is fine for winning, this system was thought to allow player actually BERSERKING (ie. killing everything) through late levels, with all those very high damage and movespeed etc. bonuses.