I just published this podcast and post and thought it might be a good thing to bring up for discussion here. It came to me when I was listening to a podcast and wanted to leave a good review for the episode and realized I couldn't because it was on Spotify, then I went down a thought rabbit hole :)
Like most things, I suspect the value of exclusivity is basically "it depends". Your concerns about exclusivity are valid, but there are benefits to exclusivity. For example, having Spotify purchase exclusivity for a podcast tells everyone that the podcast was worth Spotify purchasing them. You're not just getting users from people browsing the Spotify app. You're also getting users from people valuing your podcast higher because someone else valued it highly.
Thanks for commenting, you raise a couple of very good points. It does suggest a level of trust when a known source says 'we value this'.