Most of my peers (in the socialist republic of EU) work 4x week to minimise the money that gets wasted in taxes. Often times when people progress they just ask to keep the same salary and work less so there is less taxable income for the taxman to steal (and rates are not progressive: if you earn even a penny over a certain amount you pay significantly more taxes on your whole income).
We are all contractors to minimise taxation so the company gets to pay 4/5 days and gets most of the productivity anyway.
For managers it's a no brainer, for employees, I don't think the extra day of freedom is worth the loss in pay.
I guess in a stricter company we would be punished for not being as productive on friday.