Let me put it to you this way:
If they had released the pre-release as an official update, all the whiny kids who don't know what a beta means would flood the minecraft forums crying about how broken it was, the first pre-release had some game breaking and map destroying bugs, which have been fixed, the list of bugs has been severely decreased and the official release will probably come in 1 or 2 days, then we'll have a mostly bug free playable game for a while, and those kids won't complain as much.
Personally I'd like pre-releases every week before an update, it's a great way to find bugs and try out new stuff before the masses, and we help them release a better product
