Well put. Not to mention the disconnect between market rate ("the average of salaries for similar positions in Market X") and individual rate ("the amount you can get in Market X given your skills and experience").

Fundamentally, you are worth whatever someone will pay you. If you can negotiate an offer for 20% over market rate, congrats, that's your personal market value. Conversely, if you're currently paid 20% under market value and can't find another job, the same applies.