It's not forcing you to use special weapons. It's forcing you to use his Z-Saber with its special abilities/techniques. It's still removing abilities to create emphasis on a certain attribute (special weapons or Z-Saber) and it makes ZERO sense for him not to have his buster, which for all intents and purposes was as strong as X's. Also, with MM9 & MM10, you could argue that they removed the charge shot & slide to make him different that Blues, and later Blues & Forte.
Zero was nerfed, just like Rock was, probably to make the game just a little more challenging. Like Zan said though, there's no reason not to make it an unlockable later on.