1) I can code offline. I must admit, that i have to regularly unplug my Ethernet cable to avoid being to distracted. An online editor would encourage just tabbing into a site with distractions
2) I can code offline. If I don't have an internet connection for some reason I can still work. This is especially important for people with instable connections.
3) I doubt an online editor will be as customizable as emacs
4) I can run it as root, if necessary. I wouldn't dare to run my browser as root.
5) I see no advantage concerning the use of one editor on many platforms. If I copy my .emacs to any other (Linux) box I have the same functionality everywhere.
Coding offline is also somewhat important for me.
I will typically open multiple ssh sessions to a remote machine and edit/test the code there from the different sessions.
As the saying goes, "I do it in production" ;)
What you have to understand about my editor (emacs), is that it sucks.
There, I said it. Emacs sucks.
But emacs sucks less than everything else -- in part because it sits about as close to the bare metal as possible. And also because of emacs lisp, which I hate. And love...