I ignore subscription changes, but of course I want to be notified if someone adds me to another report or creates a report with me as cc. Both things fail to happen, which probably is not expected by people cc'ing me.
Original report: https://phabricator.wikimedia.org/T107552
If it was an actual mention, you'd presumably get the mail under the "comment" rule, although it's possible the user in question has also disabled comment mail.
I do think it's reasonable to change this preference to mean "subscribers other than me change". There's a similar issue where you'd presumably like to be notified if someone kicks you off an object before doing sneaky things. >_> <_< (See T4776.)