That's more energy efficient, sure. But it sets a lower boundary on the power draw much higher than a normal Pi. A Pi plus a single external HDD draws 12W at the socket, according to my measurements. A PC CPU draws 4-5x that, just by itself. The other components on the motherboard need power too, even if you use integrated graphics or no graphics at all.

A PC only becomes more power-efficient if your load can't fit in three or more Pis. For plenty of uses, more than two Pis are an overkill.