Okay this is one of those out of the box thoughts as compared to reality. Windows by itself, and current systems, are low power usage unless we are discussing Servers, Server Farms, etc. (which that article would QUANTIFIABLY impact the overall power, heat and noise consumption on that scale) mainly but that was before the constant multitasking and 'less user maintenance' approach to the current Windows editions. These now demand MORE constant 'in the background' use of the HDD, like checking for patches, scanning files for Viruses, checking for new Tweets / FB entries etc. On the larger scale (as I mentioned) business was worried before on the costs because the 'machines' sat 'idle' while everyone was home 'asleep', that changed that now all these systems are Interconnected and available 24x7x365 and used 24x7x365 now too. So the need to 'idle' or 'power down' the drives has given way to 'constant accessibility' needed by them.
I really wouldn't concern yourself with this for your home simple PC, this is really not relative (example: by doing it you save .00002 Watts usage in a month, does this really impact your electric bill?) but for some home business people whom may have a mini-server or such they may benefit, but things like E-Coin mining would use alot MORE power by the GPUs then you would barely save with the HDD for example.