Open sourcing your MMO

Many single player games have gone open source with raving success. Take Quake for example. When Id Software released the Quake engine as open source with Quake 3, people went nuts with it. How many mods and games were created after that using "The Quake Engine"?

So the question posed over at Stroppsworld's blog is why can't we open source our MMOs? Stropp makes some great points for open source, against open source and a common ground that may be a viable alternative. It seems he's covered all of his bases and asked the questions we'd need to ask for a project like this, but wants your feedback on his ideas.