The ads are being pushed to your iPhone by Google, who's motto is "Don't be evil." Ads I'm ultimately paying for in my data plan because it pushes them to my device without my permission, hence the reason I won't use the app. They might not be very big data-wise, but several hundred of them over the course of a day DO add up. While I understand the need for advertising, I don't think I need to be seeing an ad everywhere I turn. It's the same reason I don't watch YouTube on the web on the desktop. Too much garbage distracting from whatever it is you're trying to watch.
What is evil about subsidizing their services with ads ? Are you suggesting they should offer payed subscriptions to YouTube instead ?