Obviously, when people are rewarded for their effort it encourages them to refine and improve their software. I'm all for plug-in developers selling commercial GPL plug-ins and it only seems fair that wordpress.org would promote them like they have started doing for commercial GPL themes. Themes seem to get most of the attention but many website are built around the functionality of one or two plug-ins.

I guess a good question would be: What's the harm of creating a commercial plug-in page on wordpress.org?