When a spam control solution thinks legitimate email is spam, what does it do? It might delete it outright; it might send it to a junk folder. Admit it: If it goes to a junk folder, don't you actually delete it along with all the spam, whenever you finally get around to it? There's just too much to deal with! And have you accomplished anything to fix the problem?
The fact is, these false positives will cost you business. It may not happen often, but who knows how much business that lost email may represent? And what effect will that have on your or your organization's reputation?
Doesn't a 100% effective solution sound better?