Yes, their inventory is smaller at the moment, but they also show ads in News and Stock apps (note that those ads are personalized and don't have an ATT prompt by default.. so Apple apps don't have to play by the same rules as others). My guess is they will continue to expand inventory to other Apple apps (eg Maps, etc), but whether they try to build a full-fledged ad platform (where you can use Apple to buy ads in 3rd party apps) is yet to be seen.. they'd definitely have to overcome a backlash if they did that.

Where do you get ads in the Stocks app? At least I don't. Though the yahoo finance news stuff is rather useless.