This is not practical if you are a content creator. If you use same npub everywhere, your followers on Amethyst will also be your followers on zap stream or any other client.
If you use different npub then your current followers will never know that you also stream on zap stream or post pictures on olas or use any other client.
Google sign-in is different. It only allows you to login but nostr is different. Your whole identity, your followers, your notes and other stuffs, everything is linked to your npub. And you take it with you when you move from one client to another.
I think this is a superpower of nostr and I love it.