Strawman?
The proposition is this: Halve the mechanical benefit of X to prove whether a player is taking that option just to optimise and not for the roleplaying reasons.
So I took Fighters and Wizards as X to prove why this is a seriously flawed proposition. I see nothing wrong with my argument.
If you don't want a player having an option because you think that he's just doing it for power(or if you think it's thematically inappropriate or for whatever other reason) then at least have the courage to say it to his face instead of resorting to sneaky shenanigans like those.