It's really convenient to use the moderator-ping (
@@<userid>@<sitename>stackexchange.com) to summon users into a chat room.
Unfortunately, this tends to create duplicate chat users if they happen to have an existing chat account associated with another site. Which is a problem if the existing user has access to rooms the new one does not. Or a really clever status text. Or a coveted low-digit ID number.
Sure, I could just look for the user on chat first; but if they have a different user name on their preferred site, that's difficult... and worse yet, it's inconvenient!
I would much prefer it if the system could simply do this for me. Or failing that, provide some convenient way to merge users.