What's the point of a public option?

In the national debate over health insurance reform, a key issue is whether the government should offer a "public plan" to compete with private insurers. For example, in today's NY Times, Paul Krugman writes,

What’s still not settled, however, is whether regulation will be supplemented by competition, in the form of a public plan that Americans can buy into as an alternative to private insurance.

Now nobody is proposing that Americans be forced to get their insurance from the government. The “public option,” if it materializes, will be just that — an option Americans can choose. And the reason for providing this option was clearly laid out in Mr. Obama’s letter: It will give Americans “a better range of choices, make the health care market more competitive, and keep the insurance companies honest.”

It seems to me that this passage, like most discussion of the issue, leaves out the answer to the key question: Would the public plan have access to taxpayer funds unavailable to private plans?

If the answer is yes, then the public plan would not offer honest competition to private plans. The taxpayer subsidies would tilt the playing field in favor of the public plan. In this case, the whole idea of a public option seems to be a disingenuous route toward a single-payer system, which many on the left favor but recognize is a political nonstarter.

If the answer is no, then the public plan would need to stand on its own financially and, in essence, would be a private nonprofit plan. But then what's the point? If advocates of a public plan want to start a nonprofit company offering health insurance on better terms than existing insurance companies, nothing is stopping them from doing so right now. There is free entry into the market for health insurance. If a public plan without taxpayer support would succeed, so would a nonprofit insurance company. The fundamental viability of the enterprise does not depend on whether the employees are called "nonprofit administrators" or "civil servants."

The bottom line: If the goal is honest competition in the provision of health insurance, the public option cannot do much good but can potentially do much harm.