Doesn't knockdown break engagement? So it has some use there at least for safely disengaging. I was also under the assumption that getting up from knockdown uses half of the target's movement, but I perhaps might be confusing that with some other (D&D) games. Personally I'm fine with knockdown in TB as is, however I think that being knocked down needs to come penalties to defense against strikes made in melee, of course, offset by bonuses to defense against ranged attacks.