It's not so much a "requirement" as a polite request. Two people politely declined, around a dozen or so (including all of the most prolific and valuable contributors) readily agreed. I don't want to complicate the developer documentation and distract people from writing code by adding that fact to the developer documentation.
Which, I suppose, definitely proves one of the points against such contributor agreements. Perhaps I should reconsider and add a mention about that to the developer docs.