What does bitcoin do though? You cannot tell me that all that power is just going up in smoke....
then why do you think we expend a lot of energy and fuel to dig gold out of the ground? is it necessary?
you can see mining like finding serial numbers for Bitcoins (although that is technically not entirely accurate, but may be a good short comparison or explanation for newcomers) in order to limit the money supply in order to make bitcoins valuable.
Primecoin is a nice experiment, but it's yet unproven if those chains of prime numbers provide any additional value.
I agree energy expenditure required by proof-of-work is not the nicest thing about Bitcoin, but the only alternatives known so far to reach consensus in a decentralized system are proof-of-stake and a web of trust. Both have other problems.
In theory, a centralized digital currency and transaction system would require only a database and could be much more energy efficient, but I'm not so sure in practice when I look at the big banks and all their COBOL crap they're still running on inefficient, legacy mainframes in giant data centers.