It'd be cool if there was a spotting mechanic, but as far as I know, kneeling does nothing more than improving your accuracy (it does help a little if you are behind a low elevation cover piece, but it will also probably block your shot while since your head and shoulder are exposed, the enemy will likely still make their shot..).
Partial cover has very little impact in XCom, if you're seen, you're a good target, unless there's some funky firing mechanic happening (like when standing just out of a UFO and trying to hit someone diagonally by the door)