Such a judgment requires that the company evaluate you carefully and in terms that are relevant to its business, not in terms that are important to someone else. Once the company decides what you are worth to it, negotiations can proceed in a pretty rational way. A smart company would never pay you based on anyone else's judgment of your worth, because that worth is relative. That's why there is no reason under the sun to divulge what your last employer paid you. It simply should not be necessary because it is not relevant.
Salary surveys can hurt employers.
Now, some may argue that a company must know what other companies are spending for a given constellation of skills, if the company is to be competitive. The use of human resources salary surveys can actually hamper a company's efforts to be competitive because these surveys drive job offers toward the mean. Junk logic can hurt you. Surveys aside, there is a reason why you personally should not divulge your salary to a new employer. Actually, there are two.