The aperture can be changed, and really easily, too. The only tools you need are a 1/16ths punch and a small hammer. Personally, I think the half-cut aperture was put there to make the sight fit under optics (ACOG comes to mind) better; they just came up with that "horizon line" stuff to be able to pass it off as an "improvement". Keep in mind, that that is only my OPINION, and is in no way a statement of fact. Ok, having said that...
I like the XO same-plane aperture a lot, myself.