When I used spam filtering I was receiving perhaps ten spams for every valid mail. This made it very difficult to scan my filtered mail. There was just so much of it.
For instance: IF X email is in the spam folder AND contains any of the words (Cialis, Viagra, Rolex, etc...) -> Move it immediately to the trash.
This cuts down the number of emails that stay in my spam folder by a huge margin, and makes it much easier to check for false positives.
That being said, I no longer even bother checking. I've been with Gmail for 5 years and I've never had a false positive.
That being said, you still have a point.