Great workaround for a quick peek at total messages. However, I want to agree with whoever said this is definitely a bug and not a planned change. If it was a planned change in functionality, it wouldn't sometimes work and sometimes not, and there would be no way to restore it by quitting and restarting the app. Mine has functioned much better since I rebuilt the mailboxes with Onyx. It still loses the count every so often, but a reboot always restores it. It is still a pain, though, and pretty shocking that Apple would allow this to linger for so long. Once upon a time, Mail was a dead app. But since mobile, it has been given new life across all platforms.
I now also have a new and much more annoying Mail issue in another thread that no one has responded to yet: "Include selected text" option not working since last Yosemite update