My experience is similar to freyr's. I've had Friday-from-home periods where usually I'd end up not doing much. Sometimes this was my fault; it's just too easy to see it as an early start tothe weekend. But at other times it was because people/code/decisions I needed were not made because employees in the office also seemed to have much lower output on Fridays.

And my experience with coworkers is no different. I actually checked commit histories at some point to confirm that indeed most of those who worked from home on Fridays didn't produce any significant output. The sneakier ones would commit stuff, but clearly 'little stuff' that I sometimes suspected they had ready on a Thursday specifically to show something on a Friday.

Of course, you're completely correct that this doesn't mean that this is always the case. But if I were an employer, based on my experiences, I'd be hesitant to allow people to work from home on Fridays. Not in general, but specifically only on Fridays.