Taking 10% of the income from someone who makes $20 million a year probably won't decrease their productivity. Giving that $2 million to lower income workers so they can afford to send their kids (or themselves) to college, could (and probably would) dramatically increase their productivity, thus increasing the productivity (economic output) of the society as a whole.
Of course, the person you're taxing might be more efficient at allocating resources (e.g., he would invest the $2 million in a highly successful venture, or use the $2 million for scholarships for certain students whose education would increase productivity more than a random sampling of lower-income college students).
But I don't think most rich people are super-efficient allocators of resources, so I think Randfan has the right of it.