The same could be said about $mircdir though, unless the ability to use a mirc.ini from anywhere you like is removed and you're forced to have it in the appdata folder.
If an administrator installs mIRC to c:\program files\mIRC\, then a non-admin would probably not be able to use files in that directory.
I think the only way to ensure your settings are stored in a writeable location is to store them in appdata no matter where mIRC itself is, in which case an $appdata identifier would be useful.
