Why the arguments for Dual Spec are bad

I would say it depends on how duel specs is added.

As an example I wouldn’t be against it if

It was 10k to get (estimated cost people who respec a lot will go through in 6+months)
You could only respec in a major city (no changing whenever you want but still faster than respec as the talents are already chosen and you don’t have to go to your class trainer)

I’m not saying I want duel spec BTW, just that if it went into game I would prefer this version.