As someone who's spent quite a bit of time talking to actual lawyers about the issue, I can say with a high degree of confidence that your friend Beagle should not be relied on for legal advice.

The GPL is explicit about what constitutes a derivative work, and themes/plugins are derivative work. The GPL does not prevent you from creating and selling themes that are not GPL licensed, but using them in conjunction with Wordpress (or any other GPL'd software) would be a violation of the theme's non-GPL license.

I understand that there's a lot of friction and frustration on the part of people who've built business models around restricted-distribution themes and non-GPL themes. It's painful. But the fact that it's annoying and frustrating doesn't change the fundamental nature of the GPL license. If someone wants to leverage a ginormous existing market that uses GPL software, playing nicely with the GPL is necessary.