I don't think I've seen an armed kick in any of the manuals where the weapon wasn't first intercepted, checked, or controlled by the kickers weapon. It makes the kick a single time counter, instead of just a dangerously exposed attack, and keeps you even from "helping" your opponent hit you.
A kick to the hips will lever someone forward, but if it's a thrust, it will also push them away. A thrusting kick to the stomach will push someone away pretty well also, without levering them much, and if they're wearing a solid breastplate, it won't cause them to bent forward at all.
I train in harness quite a bit, and yes, it does give a bit more mass and momentum to strikes with the body (I hesitate to say unarmed strikes, as the armour is quite a weapon). It really doesn't slow them down, you'll just get tired sooner. Even with no weapons, someone in harness is quite a bit more dangerous, especially if they know how to use it.
