Proof of work (= energy expenditure) is the price to pay to generate a trustless system. Therefore, IMO, the energy expended by PoW is an interesting offset to human malfeasance, conniving and greed.
Now, there might be other, more efficient ways to offset human pathology, but bitcoin's implementation is successful enough, at this time.
I wonder how then the BTC differs from gold as currency. The same proof of work was present then, as gold is hard to mine (at least in Europe, before they came to Americas.)
In this aspect gold is better because it is useful (for jewelry, electronics, science) and it does not depend on social acceptance to have value. BTC has no inherent value, and thus all the work that went into minting the BTC hinges on acceptance. As you can see, BTC combines the worst characteristics of currencies - it is hard to mine, like precious metals, but it has no inherent value, like paper money.