The major problem is that you can't control the behavior of a podcast client. Some of them will cache you media files "forever", unless you explicitly change the guid. But changing the guid has negative effects on some clients too, as they will re-download the episode because they will see it as new, thus creating duplicate entries.

This is the reason why you just cannot create dynamic ads if you want to stick to the standard RSS+client approach (like iTunes).

Some of the podcasters have started actually building their own native apps to have better control of the content they emit, but it comes with reduced visibility. This is why most of podcasters still release their stuff through iTunes, but invite the listeners to download their app.