Yes, the medium tag is a recent addition, but all the other types have a few feeds using them. Blog and Newsletter have zero (based on podcastindex API results), and I don't see it changing soon.
My understanding is that a "podcast" is loosely defined as an RSS feed with a media enclosure. By that token, newsletters and blogs are not really that on a feed level. I can see you might have some episodes that contain media, but rarely the whole feed.
I can't find discussion around the inclusion of these into the podcast namespace (and that could be due to my own lack of search skill), but I feel these medium types add noise and don't add value.
I am more than happy to be shown the error of my ways though, which I what I guess these discussion threads are for!