JUpdater does not work for components if their XML update element did not explicitly include a folder and client_id child node. This is undesirable, because it is always folder="" and client_id=1 for components. The workaround is to set these values as defaults in JUpdaterExtension. Plugins, modules and templates don't have a problem, as they do specify those elements when necessary.
This pull request is a follow-up to the Joomla! CMS tracker item #27023 (http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=27023) and the related discussion on the Joomla! Dev General list (https://groups.google.com/forum/#!topic/joomla-dev-general/R4g0-Q2pWXk)
JUpdater does not work for components if their XML update element did…
… not explicitly include a folder and client_id child node