Lots of folks claim they don't. They're untested, or unimportant, or whatever else. So the Urban Institute, as august and respected a think tank as DC's got, decided to run the numbers. Here's their conclusion:
In this brief we conclude that, absent a single payer system, it is not possible to achieve universal coverage without an individual mandate. The evidence is strong that voluntary measures alone would leave large numbers of people uninsured. Voluntary measures would tend to enroll disproportionate numbers of individuals with higher cost health problems, creating high premiums and instability in the insurance pools in which they are enrolled, unless further significant government subsidization is provided. The government would also have difficulty redirecting current spending on the uninsured to offset some of the cost associated with a new program without universal coverage.
