Peaks & Aerodynamics

This might be a n00b question, but I'll ask it anyway:

Why do dualsport/off road helmets have those giant peaks on them when road helmets don't?

Do they serve some sort of purpose? I'd think having a peak that could catch on things when you come off would be a bad idea, and you'd come off much more often when you're riding off pavement, wouldn't you? 

Wouldn't having a big peak on a helmet catch wind and tire you out on the road as well?

I just watched Ewan and Charlie do their Long Way Round and noticed the big peaks on their helmets and wondered why they wore those when they could have had something more aerodynamic and safe.