It comes down to:

1. Onboarding (user does not have Nostr)

2. UX (managing multiple keys, signing)

3. Features that can be built/make sense to build on Nostr for non-social media use cases.

The third point is where we're likely to see more hybrid models that are not pure Nostr-native clients. For hybrid models you need server-side authentication and one or both of client-side authorisation (ie: signing).

Delegation tokens may be one way to unify the flows on the server-side.