It's not whether 'vi' is available (it usually is), but whether it is behaving properly. Situations do occur where any fullscreen editor will fail and where 'ed' will work as expected. And at such moments, even though you don't know the full extent of the power of 'ed' you will be happy to know how to change a line and save the config file (and actually quit 'ed'; that's harder than it sounds)

An editor in need is an editor indeed!