Actually, robert, the Mongols didn't wipe out entire cities because they felt the people were useless (or, rather, not entirely, or even mainly, because of that); rather, they did it as a psychological tactic (and an extremely brutal one, I might add): when one city refused to demand to immediately surrender, it would be completely annihilated. The worst the insult to the Mongols was, the more complete and brutal the annihilation; for example, when the Mongols attacked the Persians and defeated them, the capital city and every single city that gave aid or refuge to the fleeing Khwarazm Shah was destroyed to the last living being. It was the same in most enemy capitals and strongholds; the idea was to remove the slightest thought of resistance by showing that even TRYING to resist (even when you are an independent nation) meant complete annihilation. Thus, when a lord of a city tried to resist the Mongols toward the end of the Great Khan's campaign, the citizens would often times revolt, string up the lord and open their gates to the Mongols.
Still evil, yes, but they weren't as ignorant as you make them out to be.