If the mushroom makes its victims climb trees and explode in order to spread its spores, doesn't that mean the mushrooms will shortly crop up all over Portland? And as only a tiny fraction of the population are vulnerable to it, there's no real reason it wouldn't end up being used in food again.
What if another Bauerschwein decides to, say, put the mushroom extract in the water supply?
It could be that putting it in the water would dilute ifs effects enough that it wouldn't be of any use as an anti-Blutbad "weapon".
Rosalee says it's only harmful to Blutbad when cooked.