Being able to post recommendation events as a marker of app compatibility would actually be huge, and I've never thought of it before.

Because the reality is, not all Nostr apps are interoperable, at the very least because they don't all address the same use cases. A recommendation event would make it possible to build a network of compatible apps. A service like NJump could use that information to provide more intelligent suggestions when opening a Nostr link, for instance.