glibc 2.21 was released upstream three weeks ago, and is now doing the usual stability-commits to the stable branch dance. The worst of the worst bugs are generally found and fixed on the stable branch within the first month or two, giving us plenty of time to get a happy glibc 2.21 for vivid.
There are a number of features and performance improvements in 2.20 and 2.21 that we (and customers) want which would be prohibitively time-consuming to backport, as well as being error-prone if we did, since they'd be tested only in Ubuntu, rather than by all Linux distros.
Also, speaking of backports, moving to 2.21 allows us to drop the (massive) IBM branch patch that we're carrying, as all of IBM's changes are now upstream.
glibc, of course, promises and delivers a stable ABI with every new version, so there are not any transitions to be had, nor any predictions of compatibility issues (any of which would be bugs, not features)
Ack. I like having things upstream. Approved.