In my campaigns as the danes and the english, I have made good use of merc mounted crossbows. I basically only use them to kill the byz katanks. If the byz attack with katanks on their flank, one unit of mounted crossbows can severely diminish one or two units of katanks before they get to my lines.

Of course, now my English billmen with valor upgrades from Mercia and weapon and armor upgrades from the Iberian are WAY more than a match for any cavalry that they can latch onto, making the mounted crossbows much less important in my armies.