Interesting! I tried to switch for 4-day work week earlier when I worked in consulting firm and the excuse was that it's harder to allocate me to a project.

I asked that isn't four days easier to allocate than five days as it means less allocation? Would it be easier if I would work 6-days week? Never got answer.

Most of the excuses are bullshit. The truth in consulting business is that they don't get as much money from you (even if you cut 1/5 from your salary).