As an advertiser that sounds like a nightmare to me. We want to be able to push our creatives out to a huge mass of users instantly, track whatever performance is relevant to us, then iterate on that to try to improve. Having to vet individual placements on apps would be a gigantic waste of time and I would not do it. Maybe for some niche markets it makes sense but I can't think of any.
Vetting the apps you want to advertise can also mean time spent - but knowing what are the apps where you are displaying your apps and even who are the type of users being the ad - I for one feel this is a more powerful approach.
But yet again - I'm not a big advertiser spending huge money on it.