While I believe that a market of one is no market at all, I also firmly concur with Dave Rosenberg's suggestion that open source companies that spend all their time competing with each other are wasting the world's precious supply of oxygen.
If you work for an open source company and your team is focused on trying to beat other open source products you are doomed to economic failure. That kind of focus means you can't see the big picture and you should just return the money to your investors so they can invest it in companies that have bigger goals. It also probably means that your company is fueled by ego and your investors should probably take the money away from you before you start building Lenin-esque monuments to your engineers for refactoring a project ten times.
Bingo. As Dave notes, most open source companies are not doing enough in revenues to bother competing against them. That will change two or three (or ten) years from now, but it's the case today.
I've never once competed with an open source project or company in a single deal that I've done. Never. My competition is the 95% of the market that doesn't use content management because it's too complex and too expensive. For that other 5%, I compete with Microsoft Sharepoint, Documentum, Interwoven, etc. Never open source.
Open source is the insurgent, not the incumbent. And for insurgency, there's all sorts of room. That's why I'm happy to profile Magnolia, Drupal, and other "competitive" offerings here. There's just too much room in the market to worry about competing amongst ourselves.
Which, btw, brings me back to my initial point: if a market only has room for one open source vendor, it's not a market worth competing in. I'm not sure why VCs don't get this, but I refuse to believe that the first open source vendor in a market is necessarily the best/the ultimate winner. The market is, or should be, too vast for any one open source vendor to consume all of it.