Not really -- a helmet protects you from head injury. Is a car seatbelt useless because it doesn't do much for a side collision?

I don't ride much, but one time I was out is the rain and had to swerve on a intersection that happened to be made of cobblestone, and ended up going down hard on the slick stone. Had I not been wearing a helmet, I would have had a severe head trauma.

That type of accident is pretty common, and wearing the helmet avoids lots of needless suffering.