The New York Times editorial from yesterday provides one of the most concise summaries I've seen for the reasons we need the public option as part of whatever emerges from our our healthcare reform debate.  Take a look.

Insurance companies aren't inherently evil.  They are important components in the way we deliver and finance medicine.  But, left to their own devices, there are many reasons and ways for them to avoid providing universal coverage.  That's the simple truth and there's no getting around it.  The public option seems to me to be the best way of dealing with that reality.