I think signing in with a node is essentially different from communicating via a randomly generated one-time-use nostr key By signing in with a node, you can send notifications to my account with confidence that I will see them the next time I log in, and you can log my sessions With a one-time-use nostr key, if you try to message me, I won't see the messages because I never sign in with that key again, and if you try to log my sessions, you'll only see one session

But you have to specify the node destination that you're purchasing for, plus another separate identity (which is often not one-time use) so really you're giving up more pieces of information.