Is it really? Even just extra 10W over a month is just under 7 kWh and might approach a dollar[1]. Spinning 3.5" HDDs from time to time is especially costly and they often spin down when unused for a long time. Load spike might also make CPU freq scale up and consume more power. Same with bandwidth, you'll incur ISP wrath or saturate it (or keep wasting most of that free space), even 80 Mbps[1] is 'just' 36 GB / hour and under 1TB per 24h. And that's without including overhead from Filecoin, TCP/UDP and IP themselves.

It sounds lose-lose-lose situation: keep wasting your space or saturate your bandwidth or waste power on keeping HDDs spinning all the time due to accessing them every few minutes. It may vary with your usage patterns (if these disks are already always spinning and you have REALLY good bandwidth or two separate lines) but still, seems barely worth the effort.

And why couldn't it be optimized in a purpose built machine? Cheap Tibetan hydro and free cooling (altitude), low power CPUs, fast internet, fast switches and routers, 10 gigabit lan, many HDDs and SSDs (dug up from the Western trash piles even, it's not like failing once a month is a big deal). Chinese already have ASIC for bitcoin and this seems easier to make than that.

[1] I have no idea if I'm generous or stingy on USA power prices and network speed.