I also use reading filters, and only part of the filter name shows in the tab title, so if it was after it wouldn't be visible. Yahoo! Mail puts (xx unread) at the beginning of the title for that reason.

I probably wouldn't use it unless I could turn it on and off on the fly (I spend longer catching up with what was posted while I was AFK than waiting for new stuff), but I'll vote Yes as I'm sure some people would use the heck out of it. It should definitely be opt-in, too.