That's a good point, Todd. Rotwurm has to actually win the match for the FF to come into play. I always figured it would happen in a tag match as part of Zombie Nation since it become a +4. In those cases, however, it's turns out it's usually Noxius winning with the Outbreak.
That team is a case of the whole being better than the sum of the parts. I don't know about others' experience, but the Zombies working together are a factor in my tag ranks.