I like to use a weed guard on bass and pike flies because there are usually a lot of snags, floating weeds and gobs of algae in the places where I fish. Aside from the aggravation of botching the cast by getting hung up or getting goop all over the fly, it pretty much always spooks the fish if you get snagged on the cover that they are hiding in.
I do have trouble with the materials fouling when I use the mono loop weed guards, and they don't seem particularly weed proof to me anyway. I don't seem to have fouling problems with the super hair weed guards (a short clump of super hair stiffened with head cement), but they aren't very weed proof either. I think the wire type would offer the best weed protection if they work as well on flies as they do in non-ff applications.