Great question! My 2 cents: our core mission is to democratize education, and although the car itself won't make or break Udacity, I think it can contribute dramatically to our core mission.
How so? We have students all around the world (almost every country is represented in our DAU) who will never have access to a car outfitted with hardware and sensors (easy $125k), never mind the costs needed to get a permit to get on the road ($50k!). Being able to contribute code and see the results run in real environments (ask Sebastian what he thinks about simulation!) could be a huge advantage to students around the world in their quest to jump into this industry and get credibility.
Speaking of credibility, we want to prove to the world we really know what we're doing, and that our curriculum is truly legit.
tl;dr: We hope that by open sourcing our car that we can give opportunities to students around the world who otherwise wouldn't have it.