In addition to energy, the biggest reason I no longer use old desktops as servers is the space they take up. If you live in an apartment or condo and don't have extra rooms, even having a desktop tower sitting in a corner somewhere is a lot less visually appealing than a small NSA or mini-PC you can stick on a shelf somewhere.
Tastes differ. I personally find the 36U IBM rack in the corner of my apartment more visually appealing than some of my other furniture, and consolidating equipment in a rack with built-in PDUs makes it easier to run everything through a single UPS in an apartment where rewiring is not an option.
