saving 10w of power saves ~1KWH of power (currently running between$0.10 and $0.30) every 4 days. so over a year of your computer being idle you could save between $10 and $30. (although if your computer sits idle for an entire year you could save a lot more money by not buying it in the first place)
so I don't buy that saving the electricity is an end in itself.
for portable devices that reduction in power can mean that your battery lasts much longer, which is the thing that you really care about.