What is the motivation for a graduate student to help to develop a professor's MOOC?
Why do anything that's not strictly required to graduate? For the experience. Because it sounded like fun. Maybe they're interested in teaching and really excited about the opportunity to develop a curriculum from scratch? Maybe they're excited about the pedagogy behind online courses and want to learn more? Maybe it's a rare opportunity to try something new that most academics don't get to do? 

If you only do things that are strictly required to graduate and publish, you'll miss out on a lot of interesting experiences, and you'll be less interesting to a hiring committee than someone who went above and beyond seeking out experiences to develop as a researcher and teacher. The goal with grad school shouldn't be to just publish and graduate. While it is sufficient to just publish and graduate, it's hardly a good way to make the most of your time in grad school. Go for the exciting teaching project. Volunteer to help organize that conference. Apply to that interesting summer school. Get involved in that intriguing research project. If an exciting opportunity comes along, take it. Develop a MOOC instead of TAing yet another semester of calculus (or whatever grad students in your field normally do). That's my philosophy at least, and that's why I would have jumped on an opportunity to develop an MOOC.