ed is a god send. Often times, I want to edit a file but don't want to have an editor overwrite the whole screen because I need what was previously printed to edit. When streams aren't enough, ed does the job.

Another psychological thing, it's like exiting a room and forgetting what you were supposed to remember in that room. When an editor takes over the whole screen, I feel a little mental context switch that I have to sort of fight (like having to go back into your memory once you leave a room)while ed seems like less of a break in context since it doesn't wipe the screen. That's my feeling at least.