There are other power usage concerns beyond cost. For example, if a small room can't have air conditioning you might want only very low power devices to keep from heating it up.
~35 watts isn't actually all that much heat load, and that's what you get from the likes of a business-class Core i5 at the loads routing packets would put on it (i.e. basically idle). Something like Atom or AMD Bobcat is <15 watts.
Another thing to keep in mind if you're worried about power/heat is that 3.5" spinning rust can pull >5 watts per spindle. I've seen 15K drives pull close to 20. But an SSD is close to nothing and so are most 2.5" laptop spindles.
I'll concede that a small closet can get pretty warm from even lower power devices. However, as long as the CPU isn't overheating and the space is clean and dry, it doesn't matter.
Another thing to keep in mind if you're worried about power/heat is that 3.5" spinning rust can pull >5 watts per spindle. I've seen 15K drives pull close to 20. But an SSD is close to nothing and so are most 2.5" laptop spindles.