Certainly wouldn't hurt to have one of those.
Kinda just like the third brake light that (I believe) is mandatory on all cars in the US now-a-days.
One thing I would add. I have LED marker lights on my dirtbike trailer that look similar to the unit that attaches to the helmet. After a few years of being in the sun, the clear housing started getting a little clouded over. I haven't tried buffing it out to see if it'll clear up again though.
But then again, the trailer always sits outside, all year. A helmet on the other hand isn't usually out in the harsh elements that much, so if it would do the same thing, it'd probably take ALOT longer to happen.
AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC
"When life throws you curves,
Aim for the apex."