I'd rather not tie my nsec to my search engine until we get a master/managed-child key solution. Even then, I'd advise creating a new nsec—same as creating a new email/password for all accounts—which is best privacy practice. So, in my opinion, privacy-wise, it kind of defeats the purpose for now.
What if nostr:nprofile1qqsrgyzdaheuckfksqkgxz9r6qys72zh6j46f69hyzkv4a4j4vzfj6gpzdmhxue69uhhwmm59e6hg7r09ehkuef02xv6p6 added Nostr login? đź‘€ 
Discussion
An email address requires you to login somewhere and authenticate. A brand new nsec does not in the same fashion. Using multiple nsecs for various Nostr applications will become normal for people not wishing to comingle certain data.
I think allowing people to use decentralised social network credentials to identify themselves to other online services is a great idea. Go for it!
FWIW I don't believe there's such a thing as "best privacy practice" in the abstract. As with security practice, whether a given practice is dangerously sloppy or crazy overkill depends entirely on your threat model. It's important too not to let perfection be the enemy of the better.