I once read an article about the physics of fairy tales that brought up that very point. Hurt her head? It would break her neck! Rapunzel would have to wrap her hair around the bedpost a couple of times before letting it out the window, if she wanted to avoid doing some serious damage to herself.

But I think that the reason why she didn't cut her own hair off and just climb down it, was because the witch didn't let her have any scissors or knives with which to cut it.