Somebody claiming to be Snit Guckfung via Standards wrote:
XEP-0224 is pretty clear that the purpose is to send a message which cases more of an attention-grabbing alert than the same message would otherwise have received, and that how to get the user's attention is up to the implementation. So if a client gets a message with this tag it should alert the user in some way. In a MUC usually a message does not notify the user at all, so any notification at all would be more than usual. That is why it is appropriate for "room mentions", since those don't mention anything but rather indicate that a messge should alert users.
Sure, I do think people at least agree that its meant to get the user's attention more than a message otherwise would, but its use beyond that isn't agreed upon even if we only consider what's been discussed in this thread.
The use-case you've proposed is seemingly to BE the room mention, which users send directly.
Another use-case is for the MUC service to use when a message contains a mention. When forwarding the stanza to a user that's been mentioned, it attaches the element.
There was also the use-case where its meant to mark a message as important or urgent, entirely independently from mentions/notifications.
These all seem like the same case. A room "mention" notifies the user. A message containing a mention notifies the user. An urgent message notifies the user. I'm not sure I think the second case is the thing to do, but it would work fine?