It actually costs money to enforce such contracts, they need to retain an attorney to do this, and I do not see any obvious financial benefits to the employer.
One possible explanation can be found in the "organizational isomorphism" theory of organizational behavior. This theory maintains that under certain conditions organizations copy cat other organizations perceived as "successful models" in a field and thus become more an mode alike. A corollary to this theory is that managers are often clueless idiots who do not know what they are doing, but they are very concerned about creating appearances of their competence. Mimicking "successful models" is a Dilbert-like strategy of creating professional appearances while being clueless. The spread of "no compete" contracts to areas where they serve so seemingly useful purpose may be a result of such managerial copy cat behavior.
Any other thoughts?
-- Wojtek
"An anarchist is a neoliberal without money."