1) If you want to get acquired - and we're talking about a big acquisition here - it would do to go free so you manage to score a big-enough user base to get noticed. And so the bigger company is interested in acquiring you not only for your technology, but also for your users. (That said, acquisition may be a risky bet.)
2) Going free also makes sense if you want total market dominance. In which case, free is really the only way to go. The writer is speaking from the POV of a micropreneur, and it makes sense for him to retract his free plan (in fact, it makes sense for 37signals/the 37signals model to do so too).
But if you're in the position of being the next Facebook, going free and dominating the market is really the best thing to do. (Or Youtube, or Google, or Flickr).
You find lots of users, lock out the competition, and then you can figure out how to monetize.
Which only goes to show that there are all kinds of businesses out there, and the advice you read on the net really should be done through the context of your particular business and/or market.