Don't forget they still get the Ogre ability to parry while mounted.
I think most people are put off of them because, for 45 points, you get +1 WS (rider), Str (rider), T, W, A (rider), Ld, Impact Hits, and AS not to mention a Best-Of list of USR's. That's a little much to swallow when such a clear alternative is offered with the Mournfang. That said, anything putting people off from using Jervis pompous-as-hell Johnson's units is alright in my book.