Have you tried one on to see how comfortable you would feel? Make sure you try it out with your helmet on in a riding position to see where it limits your head movement.
I've worn one on highway stints that linked significant amounts of off road. As Rucksta mentioned, it does stop you from doing quick head checks, which I consider critical for street riding. I use a Klim F5 helmet, and I really have to stretch my neck in order to look behind me. Doing this also causes the side of my neck to rub against the brace, so I get some chafing too. Makes for a really uncomfortable highway riding experience.
No doubt the extra support could be useful in a fall, but make sure you can deal with the restrictions so you don't increase your risk of getting into an accident in the first place.