All communication technologies can be useful or annoying; it just comes with the territory. Consider the integration of Google+ Gmail.
As part of an upgrade to Gmail, allowing contact information to be automatically updated using Google+, Gmail soon will suggest your Google+ connections as recipients when you are composing a new email.
That might be useful when you actually have forgotten to get the email address of someone who want to communicate with, and email is the context you already are using. The “problem” might be that Google+ contacts also can send you email without knowing your email address.
Whether that is a good thing or not will depend on the context as well. As somebody who gets thousands of emails every day, I probably do not relish the thought of more messages. But that is not how many users will view the new feature.
It might be helpful to have one more shortcut for sending a message, especially when you are not responding to an inbound message.
Apparently, 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.
Also, emailing Google+ connections also takes advantage of Gmail's new inbox categories.
When someone in your circles emails you, the email will appear in the “Primary” category. But if you don't have them in your circles, it will be filtered into the “Social category” (if enabled) and they'll only be able start another conversation with you if you respond or add them to your circles.
Of course, you also have some options to enable the feature or restrict it entirely.