Well having just played a zerk here's my two cents.
Zerks are made to get in combat, if you're really outdamaging your opponent then you do nothing but lag, same as a warrior except a warrior is probably pumping a lot more damage. If you're not, then you're fiddling with haymaker which is either missing WAY TOO MUCH (That needs to be looked at), or failing, or being blocked one way or another unless you take a certain path. If it's a caster messing with you you're smacking headbutts which okay, totally work. Now if all that isn't working you try some cleaves, but lo'an'behold your opponent's weapon/shield is most likely burnproof so that was totally useless. Okay, now providing you're still alive you rage in hopes you can up your output. Cool, you're doing more damage and now you're rolling but whoops! a very heavy defense is gone so guess what, your opponent is now landing more blows so the output ratio is still AMAZINGLY the SAME! (Give or take) Unless you took a certain path and now your haymaker is useless. Cool eh? No, not at all. Zerks have a VERY slim chance of doing well, not to mention it's NIGH IMPOSSIBLE to get eq solo so unless you have friends who aren't chumps and are willing to help you're SOL my friend.
So what is the remedy to this? Well this seems to be a bit like this supposedly P.O.'d and scary dude is kinda a featherweight doesn't it? Well what about this, perhaps give all opponents an extra half round lag while fighting a zerk for all skills/spells they cast? Or! Maybe when a zerk rages the opponent gets a message like.
co flamestrike/c hell/co harm
With PieMan's Onslaught you have trouble concentrating.
You lost your concentration.
bash/trip/grapple/charge
With PieMan's Onslaught you have trouble aiming.
Your (blank) misses PieMan.
I mean because you have this crazy mofo flailing his arms and blades in a million directions as his eye burn with the flame of 1 million burning feral hides trying to rip your face off you'd be a little put off too
[Radio Edit] Now I'm not talking like every time they do this, but make it a zerk skill that only works while raged and has it's own roll.