I'm talking from a framework consumer perspective.
It seems that the knpbundles.com
(the largest) community is trying to stay updated with the master
branch, while (obviously) symfony stays on the stable realease.
And this makes confusion.
Symfony 2 is quite stable, meanwhile 2.1 seems a major update,
with a lot of changes in many components.
We should always remember one the main purpose (it is my point of
view) of a framework...allow the developer to do complex things in
a simple manner.
So, if we have an update that can break something, in my opinion
it must be isolated and well documented.
The Knp team never advocated following Symfony master for bundles
listed on knpbundles.com AFAIK. It is simply that the feature
(requested by the community) has been implemented in the simplest
way but we would obviously need a more complex one.