Re: Just another Distro
AOSP is updated by Google every Android release and Google says what goes in it. If Google for some reason doesn't want the bug fixed then it's not fixed in AOSP (e.g. IMAP IDLE or ad-hoc wifi support) and everyone has to do their own fixes in their own distros. Often these fixes never happen.
Cyanogen has stopped working on CM11 Milestone 13 so they can start working on getting their Lollipop version out the door and when AOSP is updated mobile manufacturers have to customise it before it goes into their mobiles which takes 3-6 more months. This wait wouldn't happen if AOSP were really developed in the open.
If Android were truely extensible then most of their work could be done by allowing drivers to be added with the minimum of fuss and letting phone customisations and operator customisations be installable as apps (if you uninstall them or they become incompatible with later versions of Android then they would drop back to stock Android).
There is also the elephant in the room which is Play Services which is sucking more and more things out of AOSP and making them proprietry, letting the AOSP versions rot. It's got to the point where there needs to be an open source equivalent of PS.