So overall, I don't have a great answer from real world practice, as the only products I know that integrated with Nostr that didn't have 'nostr functionality' are bolt.fun (defunct), geyser.fund and stacker.news (both have nostr functionality now). Also maybe wavlake and fountain? Any other I'm missing? Also Mutiny!

However, where I would consider nostr (if it's not a full-blown nostr app) is:
- if I need social graph for my app to function (access 100 connections per user - Mutiny case)
- if I need data for personalization and AI training (train on 1000 liked posts per user) (also personalized stream of zaps in Mutiny)
- if I need virality by social sharing (reach 10000 nostriches)

Does this sound like something saleable? 

 Social graph might be the only one that makes sense to me. But who knows, people might see differently.