Some people want to be able to run their own query service, linked to their own wikibase and updating from their own recent changes but also including a subset of wikidata items in their own query service and trying to keep this up to date with the items on Wikidata.org.
RecentChangesLinked could be a method to allow checking for subsets of recent changes, rather than changes to all items. Users that want to do this could create a page on wikidata containing links to the items they are interested in and have the updater hitting that.
As well as the change needed to the updater for a new target this would also mean there would be 2 updaters running, each with a different source. I imagine there would be conflicts with the dateUpdater timestamp stored in blazegraph, perhaps that would also need optional configuring or at least a bit of work to make a date map to a source?