Look at it this way. You are sharing a server with many other users. Would you like it if someone had an easy cpanel login and some unsavory character used that easy login to take over EVERYONE'S web site, including yours?
While it won't prevent a seriously determined (and demented IMO ) person from trying to break in, a random username, and password for that matter, will go a long way in helping to keep not only your site, but the entire server secure.
I agree that sometimes it's a pain in the butt to have to remember different usernames/passwords, but I as I said before, it's just an unfortunate fact of Internet life!