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.)