See Paul Krugman's blog here. The only reason other civilized nations can provide quality health care for everyone and spend less money doing so is that everyone pays into the system one way or another. The wider the risk is spread, the less each individual has to pay.
And now Kos is implying that mandates are simply a way to enrich the insurance companies, whereas the health plans with mandates (Edwards' and Hillary's, for example) give a person the option of buying into a non-profit Medicare-type plan. See another Krugman post here. The point of the mandates is to lay the groundwork for a universal, single-payer health care system in this country, which will cover everyone and cost a lot less than the mess we have now.