I'd argue the value of the computation is strictly less than zero, because I could do it cheaper without using Ethereum at all.
computing power is inherently valuable. the value of AWS is less than zero because you could just buy a computer and do the computation cheaper at home. duh.
Gasoline allows one to use large amounts of power at the same time, say, to run a generator, start a power plant or run power tools for construction.
Bitcoin is not useful in and of itself as the current system of transactions is perfectly workable.
On the other hand, try running power tools on solar power... (including making the huge batteries and plastics)
The value of AWS is greater than zero because spending my day monitoring my server would be exponentially more costly than paying Amazon to do it at scale.
And Ether is worth less than zero (as a computation engine) because I could develop and run the computation in less time by opening vim, even if the Ether was free.