In Grand Theft Auto 5, it is very easy to die while riding a motorcycle. I noticed if you wait a few seconds while on a bike (make sure you are stationary), your character will put on a helmet. If you were to crash, does the helmet protect you from damage or is it pretty much useless?
It doesn't protect you
Just found this Youtube video about GTA 5 Myths.
In this episode they test also exactly your question. And they see, it doesn't protect you.
It will protect you
As the video in xan's answer is shown, it will protect you.
When you search a bit for this question you'll get many videos where you can see it will protect you, other shows it won't protect you.
Just try it by your own.