What are health care co-ops?

Here is one quasi-answer:

But so far, cooperatives have been defined in the health-care debate
primarily in terms of what they are not: They would not be run by the
government.

That may make the cooperatives more politically palatable to
conservatives, as well as to some Democrats such as Conrad, who fear
that the public option may be a bridge too far. But it also presents
new challenges: Cooperatives would face potentially greater difficulty
getting off the ground and obtaining discounted rates from doctors and
hospitals, observers say.

Whether this would end up as a public plan under a different name, I cannot say and indeed it seems that maybe no one knows.  Ezra Klein tries to clear up some issues. 

I am in any case puzzled by the topic.  If, say, rescission is a major problem, why do not health insurance customers seek out health insurance mutuals or co-ops, both of which offer the possibility of greater consumer control and thus less opportunism from the supplier. 

Note that mutual banks were quite common before the rise of deposit insurance and mutual life insurance companies played major roles in the history of the industry.  So mutual and co-op forms can arise when the market agency problem is severe.  Why don't health insurance mutuals or co-ops take over the sector today for that matter?  But hey, wait, Blue Cross and Blue Shield do in fact have long histories as co-ops.

So what went wrong?  If you read the Mark Pauly quotation on p.2 of the first link, it seemed that health care customers did not in fact end up controlling the co-op (or mutual).  The managers ran the show in their own interest.  Maybe so, but then will health insurance customers do so much better controlling or influencing a government-run plan?

There is thus an unusual implicit claim on the table.  It runs something like this: decisive customers, with exit rights, cannot control a health insurance co-op.  Those same individuals, in their roles as voters, being non-decisive, and with fewer exit rights, can control a government-run health insurance system, co-op or not.

I can think of some models in which that claim is true, but I would not want to go to the mat for them. 

Here is someone else asking why we don't buy health insurance from mutuals.  It's an underexplored question.

Addendum: Mark Thoma makes some excellent related points.