"ed" is easier to type than "ex" but mostly it's a matter of habit. On some systems, /bin/ed is a separate binary but /usr/bin/ex is actually just a symlink to vim, and at that point I might as well run vim.

As to why I run "ed" instead of "vi": if I get just a few errors from some script I just copied over, I could run "ed" and the error message is still on the screen (because ed doesn't repaint the whole screen). I would run vi and screen if I expect to fix more than just a few lines.