I'm not sure why Matt says that the plugin repository has commercial plugins in it already. There's the software-as-a-service plugins like Akismet, sure. There's also at least one zero-cost-with-paid-modules plugin (WP e-Commerce), but you can't get the paid modules in the repository. I don't believe there's anything like Shopp in the plugin repository, a GPL plugin that the developer will only distribute to buyers.

What’s the harm of creating a commercial plug-in page on wordpress.org?

Exactly. There would definitely not be any harm, and I think it would even bolster the number of people offering such plugins. I'm not sure why there hasn't been the same kind of controversy about plugins as there has been about themes -- not that I want that controversy, but I often stumble across commercial non-GPL plugins (like WishList) and wonder. Now that we are getting plugins like Shopp and Gravity Forms (as well as the commercial GPL plugins that Joost is planning), we may have a demand for such a page -- and the existence of such a page could in turn introduce other plugin developers to the idea of commercial GPL plugins as a business model.