I agree - such a thing exists in the Mandriva distro.
They call it the "Cooker" which is the bleeding edge
rolling update development repository.
Mandriva has community testing open for their pre-release
candidates and that is where they get a lot of feedback
before "betting the business" on a release.
It works well if you actually want to build a community around
your product.
Community can make a lot noise about software flaws
and complain vigorously about development direction,
but in itself a community is also supportive and forgiving.

Nokia does not want such a community, obviously.
Their business + their product + their model = their decision.
That such a system does not exist shows a business model
exclusively geared toward Joe-thick-as-a-brick-Sixpack market.

In that business model there is, however, no room for mistake.
Which, in my opinion, is a blunder in this new century.