You know, I liken the class restriction on gear to a 'hack fix' more than anything. (not really a bad thing, but not the ideal way to do it)
The class restriction on gear/items is something that's been in the game forever, however it hasn't been used on anything but Tier and Lengendaries for some time now. They don't have to program anything extra into the loot system, because systems are already in place to deal with class restricted gear. Genius.
Ideally, they wouldn't have to place the class restriction on the gear. The loot system would take role/spec/class/ect into account when allowing you to roll need on an item, but for whatever reason, that can't be done easily/quickly and is slated for MoP. It's probably a complete rewrite of the loot rolling system.