Open source is the way to go. If anyone wants they can try and sell plugins already. IMHO its pointless to recommending non-free plugins. If so, people will start trying to make untold thousands off a 15 minute plugin that uses a basic API call.
That said, if there was some amazing plugin or theme, then maybe they should sell it. Considering how simple it is for even a day 1 css learner to create a template for this site, I dont see that happening with a theme.