I think that all types of heavy infantry should have a bonus against cavalry.
Is sword (or mace and axe) so ineffective against horses?
Yes, spear is more long for hit the knight, but the horse can easily be hit in melee.
I ve seen in EB that some time cavalry can run through enemy infantry(especially swordsmen) without died.
I think shouldn t be only for roman legionaries, but should be for all heavy infantry.