Ditto what Eddie said.
I wear a full face helmet on everything I ride. I have two, an HJC SyMax, and an older Shoei RF900. The Shoei is quieter.
The only reason not to wear one, and considering their vast safety advantage over open-face helmets, is if you simply must be seen. Harley riders come to mind. If you ride a Harley and come cruising by wearing a full face helmet, you're Mr. or Mrs. Anonymous. The whole point of having a Harley and riding it, is to be seen riding it.
There's a guy in town who rides a Ninja, a very fast Ninja. He never wears a helmet, or protective riding gear for that matter. He has a courier bag slung over his shoulder. He's some sort of computer type and rides around solving people's problems. When I asked him about the no helmet thing, he said that he suffered from claustrophobia. I felt like saying, hey, this isn't going into the MRI tunnel or being locked in the closet by Mom because of some misdeed. But I didn't say anything.
It's up to you, really, what you wear. I wear a full face helmet. I also wear, depending on weather conditions, a variety of jackets, ranging from a 3/4 length TourMaster, a FieldSheer racing jacket, or an AGV mesh summer jacket. All have protective pads. I'm not a fan of fractured skulls or broken body bones.
I call it risk management.