It's not just better ad statistics -- with apps like Stitcher/Spotify, the app contains both discovery and download, and also the player. When you listen to podcasts on their apps, they're able to report useful, interesting metrics to podcast producers like skip rate, bounce rate, etc. They're valuable metrics.
As far as I know, podcasts that are distributed by RSS and consumed by the apple podcasts app / any other app don't gather these metrics. There's no reason they couldn't though, because RSS / Atom have plenty of flexibility to add a field for a metric reporting callback/webhook. We could have an open convention for what goes over that webhook, maybe even some competition in providing podcast playback metrics as a service.
(... does this all exist already and I'm just behind the times?)