Google's advertiser-side ad network/tools/analytics is lightyears ahead, and I'll bet they can capture more $ from video ads in-car than actual ride $.
I find it hard to really believe that could be true. At least on YouTube, even with video ads CPM is ~$10, where as CPM for a taxi service is more like $10,000. It doesn't seem like it could be a particularly important amount of money to them.
Of course, why not do both? :-)
(ex-googler)