Helmets are required by law for motorcycles yet I've arrived in situations where the rider had its helmet on its elbow (or didn't have one at all) and the skull was cracked open like a walnut.
I've had two situations with no-helmet accidents:
Juan B Justo and Corrientes: skull cracked clean, the biker must've died on impact.
Ugarteche between Cabello and Av. Gral. Las Heras: the biker hit his head on the asphalt and blood was pouring out like water on a hose. This guy survived with mental disabilities.
Laws are there for a reason guys. Paramedics get used to these situations but you could avoid them all together by following saftey rules.