Identity in this context really means your social graph and content produced. Lets say you build a following and some great content on ZBD, but tomorrow they do something you really do not approve of. Guess what? You're screwed. Sure you can go create a new identity on another app, but you'll lose everything you built. It's the exact same issue as twitter/YouTube/Facebook etc...
If Vitor pisses me off and I want to leave amethyst, all I gotta do is login somewhere else. My data is my data.