We hear this pretty often, but it's at best an exaggeration, at worst completely wrong.

Aristocrats went at each other - hard. There's just no way to deliver a heavy cavalry charge, or to bash the Christ out of each other in a melee, that isn't going to result in deaths and crippling. Noble or not, no one is pulling his punches when his ass is on the line. Beyond that, the aristocratic lifestyle was inherently dangerous. A lot of knights and men-at-arms didn't die in their beds: battles, tournaments, and hunting accidents took a heavy toll.

With that said, the great advantage of being part of the knightly class (including higher ranking noblemen; it's too complicated to get into in depth, but suffice to say all noblemen were knights, but most knights weren't high-ranking noblemen) was that you could expect to be taken prisoner if you were wounded or surrendered. It was dicier for common soldiers, who might well be ridden down in an orgy of killing during the last stage of the fighting. A certain amount of class consciousness was probably at play, but pragmatic reasons also made taking high status prisoners attractive: namely, ransom. The amount required scaled with status. An ordinary knight would garner a handsome ransom, but a major nobleman would be a tremendous payday. Jean II, King of France, had to pay a ransom of three million écus (gold coins).