While I think themes and plugins should be treated equally, there just doesn't seem to be the same market for commercial plugins like there is for commercial themes. Have you ever seen a "plugin club"?

Like Matt said in this interview, commercial plugins are already in the plugin repository. Akismet is the only one I can think of, although I'm sure there would be others as long as they're 100% GPL compatible.

Not sure if there is enough demand pay-to-download (or otherwise commercial) GPL plugins that would warrant its own dedicated page on the plugin repository, like there is now for themes.