I'm partially with jtn on this one. Internal to any given small company you will most often ultimately be paid what you are worth to the company (or more correctly what the company can reasonably afford to pay you) regardless of degree. Performance talks and most companies will attempt to reward performance to some degree.
However, your level of education will define your opportunities if you try to move on, especially to a larger company.
But there is also the idea of getting additional education simply to increase your level of knowledge. I learn things on this website on a regular basis. Some of it I could also learn if I chose to further my education. All the stuff I learn here could be put to good use in my job. Do I absolutely need this knowledge to perform my job? Not always. Would it make my professional life easier if I had additional knowledge? Usually yes.
Be careful of the concept of 'being paid what you are worth'. If a company actually paid you and everyone else what they are worth to the success of the company the company would go out of business. Keep in mind that, with few exceptions, you as an employee are assuming very little risk. If you want to be paid what you are worth, become a consultant and take on the risks of self-employment. Generally any given employer, especially small employers, will pay you what they can reasonably afford. This is less true of large companies but it still applies to a certain degree. That's why telling a company you will leave if you are not paid more doesn't always work. You are often being paid what the company can afford already.
Keith