The transformation of insurance into a savings scheme puts considerable distortion on that model. It becomes impossible to see what payment is for expected treatment, and what is for risk assignment. The model then encourages people to use more ("I've already paid for it") and to pay no attention to cost. And it encourages the attitude that any problem should be paid for. It's hard to exaggerate the impact of these effects on health care's crazy cost structures.
The real problem isn't people who are riskier, it's people who are costlier. Someone with a prior condition isn't a risk, they're a cost. Insisting they should have "access to insurance" is simply insisting on cost redistribution. The circumstances under which that cost redistribution is a good idea socially should be an entirely different question than the efficient structure of health care payments.
These concepts are deliberately obscured by the political rhetoric around "health insurance", precisely to obscure the cost shifting nature of the project and to create a sensibility that the problems is somehow the greed of financiers. (Obscuring the cost drivers is also a goal, it's a very large industry that employs a lot of people, many of whom, at many levels, stand to lose a good deal if the system were ever seriously rationalized.) It's a very effective rhetorical strategy, especially given the abstractions around risk, but it shouldn't be confused with an actual discussion of the economic problem.
