I think for many people, most people even, this is not a rational decision. It's not about your time being worth a certain amount of value to the company and 4/5 or 3/5 of availability being worth proportionately less. Some people are offended by the idea of you wanting to structure your weeks differently in the first place & think of it purely as special treatment. They are obsessed with the idea of a five day work week and the traditional employer-employee relationship and accepting employment on non-standard terms would be giving away the employer's implicit power over the employee.