ah but there is a catch. the quieter(due to needing less power and thus cooler) the computer the more people leave it on.
you can even test this out.
Big machines-the server farms at Google-are on 24/7 anyway. so is my laptop.
Thanks to ACPI and IPMI, a properly designed computer can take its power consumption down to miniscule levels (sub-watt) when it is not used.

Which allows us to ignore it in good conscience.