My experience, though not with Nest, is that all Google devices require a Google/Gmail account. It doesn't need to be the same account and there are tradeoffs. Using the same account on everything leads to better integration. Using separate accounts could potentially increase security and privacy but may render some features unusable or more difficult to use. I use the same Gmail account on all Google devices but I limit the account to just that. I use different accounts for general email, financial services and other types of devices. That protects other services and devices in the event that an account gets hacked.