Third, Google+ conspicuously lacks its own person-to-person message system. It uses its members’ existing email accounts, which don’t need to be on Google’s Gmail system. So if you join Google+, you won’t be saddled with yet another inbox that you need to monitor, and you can reply to messages without having to go to Google+ to do so.That is a Very Good Thing. Google is doing it this way for obvious reasons which play to their advantage, just as Facebook wanted to bypass normal email accounts, for their own advantage. However, what is good for Google, in this case, is much preferable for me. For multiple reasons, including convenience and control, I detest messaging via Facebook.
(Note: LinkedIn at least makes it easy for its messages to travel via email. If someone sends you a message via LinkedIn, and you have it configured to notify you via email, the notification email is a real email--it contains the full contents of the message, as well as your connections valid email address. So you can immediately take the conversation out of LinkedIn, into email.)