Why not a public option?

Jay Ackroyd posts over on Eschaton,

The only argument in opposition to a public option is that it will lower executive compensation and shareholder value in the health care industry. Or, as atrios says, that it will get rid of the skimmers. It is not surprising that proponents of retention of the worst health care system in the OECD do not make this argument.

Jay references ...

Matt Ygelias from his post where he asks,

Moderates are very rarely asked to explain what it is about an opt-outable level playing field public option that’s so horrible that it becomes suddenly worthwhile to filibuster an otherwise good bill that will put the country on a more sustainable fiscal course will improving millions of Americans’ access to health care.

So I ask you, why not a public option. The arguments against health care reform seem to me to be mutually exclusive, such as, government can't do anything well but if we let them offer health insurance, they will put private insurers out of business.
So I'm asking you readers, why not a public option?