As a contractor that's basically what I do; the drawback is you have zero job security, and you're not really "part" of the company (which, depending of the client can be a good or a bad thing).

I think if enough people asked for it, it could become more mainstream; we spend way to much time at work IMHO.

Yeah, me too. Taxes make it advantageous if you would value the additional free time. I'd happily scale to 4 days/week now for 80% (gross) pay.