On Facebook, you can send a message to anyone from their profile, but it shows up within Facebook rather than the person's e-mail client (and it shows up in 'other' too.)
Edit: Facebook did not email me the last time I received something in my Other inbox (but that was a couple of years ago); perhaps that has changed?
Edit2: Being able to send a Google+ message from your e-mail client is a nice convenience that takes advantage of the fact that Google owns both properties. (It would be nice if it also provided a lookup for Facebook users.) I think that is all that this is, which means there is no controversy. I imagine you could already write someone a message that shows up in their email without knowing what their email address is by messaging them within Google+. If that is the case, this change is purely an added convenience that doesn't impact spamming at all.