From what i have seen from this feature, it needs the video to be implemented with AVPlayer or MPMoviePlayerController (?), and it's not compatible with a video playing in a webview (is that how IMA works?). This means that at least while PIP is active, the ads would not be interactive because they could not play in a webview.
I could be wrong, im only judging from the code i got to play with using the Beta of XCode7.
In the app i work on, PIP is disabled just because the ads can't play. No ads, no free video. I imagine that unless Apple allows some way to sneak in video ads using third party SDK's, PIP might have a very low adoption rate in general. Or maybe they will only allow their own ad platform to put video ads on PIP, im just speculating at this point.