Good question. Hmm. I think I normally prefer them to be off screen, but I have a bit of a soft spot for David Attenborough. I like it when he gets on camera and talks about something, or pokes a stinging nettle to show you up close what happens when you do that.
I prefer the narrator to be off screen if it is a nature documentary because I want to see the animals behaving naturally in their environment. If it is a geography or history doc, however, it adds a bit of interest to see the person on screen as it is more kinetic, interactive, and you have something to focus on.
I really think this can be done well either way. If I'm watching a historical documentary, I guess I'd rather not see the narrator as it would probably be superfluous, but if the documentary is actually about a researcher interacting with a subject, then it makes sense to have him or her on screen. Likewise, if the narrator IS the subject of the documentary like on Survivorman it is necessary for him to interact with his environment.
I love David Attenborough, but I think it depends on the person. He (and his enthusiasm) adds to it, some people take away from it. Especially if they make it all about them, not the subject, and do lots of stupid 'noddies'.