Being able to communicate with your fellow rider(s) is definitively a safety feature. For the past two years my girlfriend and I have been using Sena helmets when we ride together. We can talk throughout our ride, which makes the whole experience more pleasurable and sociable, and we don't have to ride side-by-side to talk. Riding single file is safer than side-by-side. You don't piss off cars or other riders by blocking the road, for one thing.
Since I am the more experienced rider, I usually ride in front. My girlfriend appreciates my ability to warn her in advance of a potential danger, such as a parked car that may be about to pull out or back out of a driveway. At a blind intersection, I inch slowly forward, and I call out "all clear" when I can see, and she likes that. There are many other examples.
And they work even when separated by hundreds of yards. When I lose sight of her in my rear-view mirror, I can ask, "How are you doing?" Her being able to respond is always reassuring.
For us, the Sena helmets do not weigh so much that they are a headache. On days when I ride alone I wear a Bontrager wave cell helmet (mostly because it has a magnetic rear light). I really don't notice the weight difference.
Although our Sena R1's don't have rear lights, my understanding is that the current Sena R2's do. Although the two lights are shaped like arrows pointing left and right, it's unclear to me if they can function as turn signals. The Sena site shows both lights blinking simultaneously. Nevertheless, I highly recommend a communication helmet, both for safety and for a more social ride.
