An example with a few reasonable numbers demonstrating why forcing insurers to cover pre-existing conditions is (1) harmful (it robs them), and (2) must raise rates. (Source)
Note: an example is not a proof; and the numbers are made up. However, altering the numbers would not alter the point so long as base conditions hold that are demonstrably accurate (for example, insurance companies determine risk and charge premiums based on average cost plus overhead). It does not suffice to say "the numbers are wrong therefore the example is wrong"; to show error, the entire fundamental assumptions about relationships between insurance and risk must be shown wrong.
Some {drivers,patients,home owners} will pay less to insurers than they cost, and some more. This may be "unfair"; but it is not immoral if a person is entering the contract voluntarily: they see the trade as a good deal. They are not just paying for the services actually rendered, but coverage, i.e., the assumption of risk by another party on their behalf.
If I voluntarily buy insurance, I understand the pool, the risks, what I'm buying, what's covered - heck, I can usually tailor it when the state hasn't gotten too involved. I can quit it if I think premiums are too high, and go to the competition (if the state hasn't killed it off).
Suppose I'm buying home fire insurance, in a pool of 1 million people with, let's say, an average chance of fire per year of 1/1000 and average cost per fire of $20,000 (sometimes it destroys the house, sometimes it gets put out and just causes some charring). Thus, the average cost per year per participant to the company is $20 (multiply risk of fire by cost of fire). They perhaps charge $25 (overhead). I agree to that and pay.
Then they are forced to let people with "pre-existing conditions" (read: their house is already burning down) enter the pool. Guess what happens to premiums?
Suppose there are 100,000 fires at uninsured homes that year whose owners sign up with the company. Cost = 100k x $20k = $2 billion, added to the former cost of $20,000 x 1000 = $2.2 billion, divided among 1.1 million, whoa, premiums shot up to $2000!
Of course, it gets worse - why would anyone get insurance with this company before the fire burns their house down when they can get it during? So costs continue to rise.
The state goons scratch their head at this, wondering why this evil company is raising its rates for the poor customers. They decide to force everyone to get insurance! Problem solved! But it was a problem they caused themselves, and likely the new rates are higher than before, with the immorality of forcing people to buy a service added in.