Freemium is an excellent model to get healthy returns, but it is somewhat limiting. Twitter clearly wants to the next billion dollar ubiquitous company, a la Facebook and Google, not just another barely profitable mid range company like Skype or Flickr.

Of course, that is a gamble (MySpace had the same dreams at one point), can lead to suckier products, and Twitter isn't exactly nailing the execution, but I can see why they don't want to charge.