How does one determine that the value of a discipline is dependent upon the boolean verifiability of it's conclusions? Why should I accept this conclusion?
In your very question you assume the existence of an ethical system which distributes values to systems of knowledge and yet that ethical system cannot be verified or tested the way that you demand all meaningful such systems should be.
The answer to why Foucault is important is the same as the answer to why you hold a belief system which itself denies it's own value.
It is curious that Foucault's questioning of other fields cannot be sustained by Foucault himself.
If I can't reliably determine that what he says is actually true or applicable to anything then there is no more reason for me to buy into his opinions than those of the Catholic church or the guy next door. Possibly less. I don't have to live with Foucault and I can't hurt his feelings, at any rate.
In your very question you assume the existence of an ethical system which distributes values to systems of knowledge and yet that ethical system cannot be verified or tested the way that you demand all meaningful such systems should be.
The answer to why Foucault is important is the same as the answer to why you hold a belief system which itself denies it's own value.