I'm already used to #nostr being a normal thing for me. I got to a website or app and see that they don't have a nostr login: "Man... I have to create a whole ass new account? This UX is shit"
interoperability, sure but it also leads to a single point of failure, if you only have one nsec for all the apps
That's a user point of failure. they could always generate a new address (in amber let's say, or their browser extension) to login into a new site. Even with this process, its still a much easier time to login ("login with new address" / one tap/click)