I think there's something to be said about the indication users of the site will get from knowing that profit-maximizing will never be a goal of the software. It may pay some salaries (as wikipedia does) but it will never sell out its purpose first and foremost as a collaborative tool.
But then I remembered some interview with the founder of betterworldbooks.com: "This idea would be best embodied in a company." In this case, maybe it's not. Lowering the barrier for a non specialist lawyer to work on the kind of cases that attract pro-bono attention is good work.