According to
versions.txt, this is not a recommended usage of $mircdir, or any of the directory related identifiers. It says that mIRC's parsing method for these wasn't updated only because most people were already appending text to them, and that using $+ would be better.
That said, I wouldn't be surprised if this wouldn't be supported anymore in the future, so I guess it's better to encourage people to use $+ instead.
