Email addresses are like phone numbers - who memorizes more than a handful these days? For years, cranking out a new email has begun with typing a few letters and clicking on the relevant contact that pops up (occasionally followed by frantic deleting and carefully selecting the correct contact). Soon Gmail users will have an even broader pool of names to select from, as Google+ connections will start to appear below saved contacts when typing out a recipient.
Naturally, these connections function somewhat differently than regular email accounts. Your email address will remain private until the moment you send someone an email in this way. Likewise, theirs will remain private until they email you back. Additionally, if someone has you in their circles, but they aren't in yours, their email will automatically get filtered into the "Social" category (assuming you still have this enabled). They won't be able to start another conversation with you unless you respond or add them to a circle.
If this sounds like more of a headache than it's worth, Gmail will have an option to disable the functionality altogether.
This integration is rolling out over the next couple of days. Expect an email from Google with details and a link to disable it when your turn is up.