Some users have activities to cross-wikis, but in this time, Echo only works in local wiki. So users could not receive other wiki's notification in their home-wiki. We already have email notification, but I think global echo notification is also good for better experience.
Let us assume that some user is highly active on two projects (A and B), but his home-project is A. So it would be nice, that on A he could get all project notifications, except for B (so the notification bar isn't so backlogged).
Would it be possible? Of course, this could be implemented in later, not together with cross-wiki notifications itself.