Because I have roommates and I have no idea if/when they are going to want to access something on my server, so it stays on 24/7. If I remember correctly, my server uses something like 150 Watts when booting up, and much less than that most of the time. I also don't pay an electric bill where I live, so I don't have a ton of reason to save energy other than generally not being a jerk.
I have a Kill-A-Watt lying around somewhere, so I'll hook it up sometime maybe this weekend and see what kind of power I am drawing if anyone is interested.
Regardless, the flexibility of having an actual PC as a NAS pretty much makes any pre-built NAS device irrelevant for my needs. Plus I'm hoping to be about ~20 HDDs deep in the not too distant future.