the only reason i could see using a wedge formation is to break troops with a charge. i know that's redundant since that's one of the principles uses for cavalry, but the wedge formation leaves too many men in hand-to-hand contact with the enemy unit. obviously, if you're using cataphracts against town guard or a pikeman's rear this is okay, but a strike to the rear against an engaged enemy with a wedge is amazingly effective in breaking it.