The latest stable MediaWiki branch is usually 3-6 months behind the one used on
the Wikimedia sites, which can be a problem when one wants to import text from
Wikipedia, use recent extensions, create a Wikipedia-related testwiki etc. The
trunk, on the other hand, is unreliable; the best solution is to use the exact
same revision which runs on the live sites, but that is somewhat inconvenient to
do manually. It would be nice to have a "live" branch or tag that contains the
same version that is used on Wikipedia.
(Or maybe it might be even better if the branch follows Wikipedia's version with
a few days delay - sometimes serious errors are found only when an update is
pushed to Wikipedia, but those are always fixed very quickly.)