Well, the idea of the Berserker class is to strengthen weak servants by making them go insane. You didn't see Lancelot or Heracles making civil discussion. Why would Kariya or Illya do anything differently concerning ME?
Well, Kariya having a sane servant would be a major advantage to him, in reality, because his best chance of saving Sakura is to summon a servant who can kill Zouken or remove the worms from her somehow. I suspect that has a lot to do with why Zouken told him to use the Berserker class in the first place....
Actually, it's because really, Zouken gives no shits about Kariya, and probably would have just iced him and taken the seals if he came close to winning, to get that wish for himself.
As a consequence, Sakura WOULD get to go free, but only because Zouken would be pimpin' out on his wish and not need her anymore.
And then the world will end.
As for mad enhancement, it is used to make weak servants strong but the cases where we do see berserkers break the rules by being strong heroes in the first place. In fact, putting them in the berserker class weakens them since they can't use their main NPs or make rational decisions. Hercules was remembered for being a cunning individual and his NP is a powerful anti-dragon NP and Lu Bu could use all of his NP's forms if not in ME. Lancelot is an odd case in which he uses his other abilities to get the power boost with little to no concequence, but he uses the mad enhancement to be stronger in physical stats to Saber.
I'd honestly like to see more ideas with berserker servants making use of mad enhancement and doing something creative with it like with Lancelot. At least he wasn't crippled by using the damn skill. As for Zouken, he told Kariya to use the berserker class to compensate for Kariya being a weak master. If he did summon a servant in a different class, that servant would be a lot weaker then it would normally be.