Wizard Slayer's do in fact make sense. They do exactly what the title says. They kill other wizards. Wizards have too many spell protections for them to go through, so they tap into the mystical realm to help them. I see them not so much as idiot fighters, but more as elite mercenaries. With their innate magic resist (the pitiful 20%), and fighting capabilities, and magic disruptance, they can just go in remove protections, disable spell-casting abilities, and capture the mage.
Sure monks are vastly superior every which way, but let's just ignore that for the moment.
http://www.gamebanshee.com/ubb/smile.gif
Berserkers don't have to be big dumb brutes. They can just be like Korgan. A member of AA, Angry A-holes.
http://www.gamebanshee.com/ubb/smile.gif Think Eminem with a can of insta-death. Remember in the kit description, that it can be either a conscious or unconscious decision to go berserk.
Kensai's I think make the least sense. They're not elite soldiers, they're the elite fighters. To me, they don't care about causes, or wars, they care about duels. That's their life, just dueling each other just for the sake of being the best. They also seem to put a great store in fairness, so the mage part totally defies this. They would be so rediculously overpowered, that the only problems they could possibly encounter is well.... gee, maybe walking through a CITY full of demi-dracoliches.
And I disagree with the original statement, because the B/M has more survivability, the K/m has no serious weakness. His A/c is hardly important, because given ioun stones, dex bonuses, and robe of vecna, his ac is already in the negatives. Toss in girdles, cloaks and rings, he can easily go into -6.
Anyways, B/m would be superior if it weren't for one item. Robe of Vecna. Set base to 5, 5% magic resist, and
decrease spell casting time by 4. So Timestops are only 5, and most spells have no time at all. If you were a berserker, you would have to decide between the ac of bladesinger (which is gotten about 15 minutes before the end of the game), or the robe of vecna (which is attainable or stealable, as early as chapter 2). If you go the path of the bladesinger chain, then you give up your ac advantage, and the Kensai still has weapon speed/+1 +1 on you. And usually, it's either CF/Crom Faeyr. or Belm/Crom Faeyr. So that's alot of power and with the small advantages over the b/m, these items severely widen the gap.
The only real bonuses you get as B/m are his immunities. However, by the time you have to worry about them, your saving throws/magic resists make all but imprisonments a problem. Imprisonment? Spell Immunity: Abjuration. No winded time.
Both of them already have so many advantages that it'd be a tough fight. Much tougher than most. Stoneskin, then a trigger with mass invis, mirror image, and then tenser's x-form, is nigh-on invincible.
better survivor than kensai mage! 
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