advantages:
1. log in once; be logged in everywhere
2. no password manager required because you have only one (or maybe a few) logins
disadvantages (I'll just keep numbering to give it unique IDs):
3. if the provider is down, you can't login
4. the provider knows when and where you log in
5. if you decide to delete your account at the provider, you can't log in anymore
6. if the provider decides to delete your account because, say, you bought a fake Nexus (or what was the story recently?) or because you forgot to pay or just because there is a glitch on their end, you've got a problem
7. if your account gets hacked, everything got hacked.
I just don't see how that tiny bit of convenience weighs up to all the issues of having a SPOF whose trustworthiness is questionable to begin with.