Other factors to the effectiveness of Cav units might also be whether they cause Fear? A lot of cav out there don't cause fear, but your more elite cav units either do or are immune (Frenzy, for example). Ld 8 with Cold Blooded is pretty handy too. Predatory Fighter is handy, but not always reliable.
I haven't played against Empire or done research on them in 8th Ed, but the Demigryphs look pretty awesome. My shop has a tourney next month and one of the players has an all cav Empire army with a unit of 8 Demigryphs (I think it was 8 and not 6 because I was wondering what he did with the 3rd Model). Anyway, not to say he will use that same list next tourney, but if I face him, what problems would I be facing besides Stomp and probably Fear?)
Also, Ravenblade said he runs a unit of 10-12 Cold Ones, so maybe that's the magic number for us? Antithesis posted about 4 Crushers and Lord (MoK on Daemonic Steed) as the best Hammer, so maybe that's the best for Warhammer (the Post backed up by modern science and technology suggests so!). In Colorado I saw a guy with hundreds of Spider Riders. (Don't know how effective an all Spider Army is, but the units were ridiculously deep in ranks.)
So I think we can all agree that either players who hate Cav are using them wrong, or not enough of them in a unit.