The public option is a threat to reform. People are not only being offered public health insurance, rather they are being forced to pick that option for their insurance by an increase in their taxes. The tax increase is a threat because they are having to pay for not picking an option. Therefore, the public is being threatened to reform to one option rather than being able to freely choose from a plethera of options.
A public health insurance option makes health coverage even cheaper. Just think what would happen if ordinary citizens had access to the same health insurance plan as Congress? We'd have a Cadillac plan for less money as opposed to a beat-up gold-plated Pinto. The cheaper car may be wrapped in gold, but the interior and engine are still horrible. A Cadillac is great all the way around. Public options are the best way to go rather than forcing everyone to get insurance or pay a fine.