"What's valuable is purely subjective" is the kind of basal argument that nobody finds convincing. It's also a far cry from a justification for wasting energy, unless you plan to take an excursion into the lands of moral relativism.

Here is my opinion: it is entirely possible, common even, for people to be just plain wrong about the things that they value. That doesn't mean they don't value them; it means that their underlying justification is predicated on transient conditions (like artificial scarcity) or is otherwise underdeveloped w/r/t to their motivations. Bitcoin epitomizes this.