Google has just implemented a change to Google+ and Gmail that will enable you to email other Google+ users without having to know their email address. If you want to send an email to somone on Google+, but do not know their email address, simply type their name in the 'To' field in Gmail, and they'll appear as one of your Google+ Connections. Google insists that Google+ contacts can't see your email address until you send them an email, according to a post on the Gmail blog:
As you can tell from the example above, these emails work a bit differently so that your email address is only shared with the people you want. Your email address isn't visible to a Google+ connection unless you send that person an email, and likewise, that person’s email address isn’t visible to you unless they send you an email.
Currently, you can receive from anyone on Google+ by default. To change this, go into Gmail's Settings, and under General, find Email via Google+. From the drop-down menu, you can select Anyone on Google+, Extended Circles, Circles, and No one. Google notes that anyone who isn't in your circles will need to get your permission to send you additional emails beyond the first one.
Do you think this will be useful, does it raise any privacy concerns, or will this simply lead to more spam?
Source: Gmail blog