With 12 TUs, you can move 3 more tiles and hopefully get into good cover where the enemy won't see you instead of kneeling behind a rock and praying. "Defensive kneeling" is pretty useless indeed. It is a flaw in the XCom aiming mechanic that the "exposed area" doesn't really matter when trying to hit something, so hiding 50% of yourself doesn't make you much more likely to be missed.
A hit (a simple roll of 0-100% compared to your hit chance) is a hit on some exposed area and it will find it the vast majority of the time, and a miss is a complete miss that doesn't even go near. There are some edge cases where what should have been a hit misses because of terrain, but they're either bugs where you shouldn't have been allowed to fire or a very unlucky situation.
For firing, "aggressive kneeling" generally gives you ~10% to hit more. It's worth doing in some situations, like when you have soldiers in a good position from where you can reliably reach a good chunk of the map (on top of a hill, on top of a transport). But for your scouts and skirmishers, it's better to walk around more and, again, get into good cover to not get shot.
"Overkneeling" is a typical XCom mistake. It makes you feel like you're doing something to help, but you really aren't. In some rare situations, a piece of terrain is high enough to hide you when kneeling but your head will poke out if you stand. But that's rare. Certain mods (like XComFiles and XPiratez) have some weapons that really benefit from kneeling, like HMGs (where the extra chance to hit is quite a bonus and a soldier manning those weapons are not really mobile any ways).
