http://www.smashingmagazine.com/2012/01/13/how-commercial-plugin-developers-are-using-the-wordpress-repository/
Just yesterday, I was looking for a plugin and there was a plugin that needed another plugin to work. And, they had an aff link in the wordpress plugin directory to the plugin needed to make their plugin work.
I thought "How clever"
The aff angle is just one angle. The README could also sell dev services, ancillary services that have nothing to do with WP. Kind of an interesting angle and I didn't realize that WP permitted this, what the whole kerfuffle (kerfluffle?) over non-GPL themes (Thesis) and all.