This concept that writing is a solo venture is reinforced by world of the novel, it is a rare book indeed that has multiple names on the cover. There is a certain (slightly egotistical) appeal to being THE author of a piece of work, other people cant mess it up if you have the only pen. That said, if you have a way to build a story as a partnership or with a group, you have access to ideas from multiple minds. Screenplays seem to more often have multiple contributors, maybe because the movie/stage industries are generally far more collaborative in nature.
What might this mean then for multiple or rotating GMs in a roleplaying game. At first the idea would seem to limit the ability to have a strong coherent plotline or set of threads that are leading somewhere. But maybe that is the exact challenge that would make it all the more fun. You can obviously default back to the 'episodic' game type that is even covered in the D&D GM's Guide (4E p13) - but I feel we could/should aim a little higher - build a logic into the game where the threads are able to evolve as a real world might between the various GMing contributions towards a greater and greater threat the party will need to deal with. The GMs leaving hooks and exposing possibilities that they and/or the other GMs will run with as things progress. Equally having several threads that nobody knows how they all relate yet, but we 'know' there is some common underlying peril that is driving things forward towards a final endgame.
It seems the common way to deal with the GM's character when they are GMing is to have them 'missing' from the adventure. I dont like the narrative implications of this myself and would like to brainstorm with a group some alternative. Off the top of my head we could have a magical solution. Maybe the party has the Staff of Doors which allows them to pass into the Shadow Mouintains. The staff however demands that one player stay inside the crystal at the top so they can return after each adventure. Thus one character is there and can see everything that happens but cant participate.
There were a few times we had to change the party because there was no brawn and the GM that session really really recommended that the party bring some along. I recall some fun GMing and some fun as a player. When Hexmark my thief/mage died (failed lotsa saves vs an illusionary fireball) it was a sad moment, but I still had my other character for the next session. If I recall at the time we also had the tradition of bringing along one of a pair of clerics as we didnt like just playing the healer. These NPCs were carefully crafted to not need to much input from us as players but were still interesting - they were run by the party collectively. I recall one was a very dim blonde who apart from her healing talents was a complete ditz.
[images are just cool D&D art from the wild internet]