Having emacs-like keybindings by default in all native text fields and text boxes (including this one) is a huge plus for OS X. In Linux, I had to write everything in emacs and copy/paste it into the field in question. Fine for emails, not so great for address bars, impossible for dmenu, etc.
I have emacs-like keybindings in most Linux apps, including dmenu. For some apps that don't support this by default, you can configure it via their gtk backend.
