Javaman

... from the intial hit... but the subsequent bouncing about could be a different story. True, the first impact may mess you up pretty badly but, after that, your belt will keep you from flinging about... if it's still working.

Pyrrho

In point of fact, a seatbelt may not only keep you from flinging about after the initial impact, it can also reduce the impact to the side, because it can impede your movement toward the door of the car. So the seatbelt can aid even in the initial hit. True, it is not ideal for this purpose, but it is wrong to say that it is completely useless for this purpose.