Community Promotion Ads would enable Stack App developers to promote their apps to other members of the Stack Apps community. Do we need community promotion ads or not?
Community promotion ads wouldn't promote apps.
These ads exist to promote sites as a whole. If you wish to put such ads on other sites, then there is nothing to stop you posting proposals on their meta sites. Most sites have a post that calls for ads, just answer it and see if your proposal is accepted by them.
Looking at it the other way, the StackApps site doesn't really need to display community ads for other StackExchange sites. People who come here often know what the other sites are.