Thank you. That would be a way to test an app or client. However, it does not vet the security of apps once you plug in your NSEC. 

Clients already have the function to log in via NPUB. You can actually log in and spy on anyone's Nostr account, you just can't post or reply from it. 

You can also use something like Amber to manage your one key and sign into clients in a protected way.

I am proposing something more robust.