If I had to choose between a $2/month increase in my subscription rate, or an in-game advert, I would choose the advert.
It could be a 3-second splashscreen for the History channel, or a building in a town reskinned to make it look like a Wendy's, or a MetLife blimp as a target drone. I don't care. Revenue is revenue, and the less that has to come from me the better.