This topic comes up frequently and there are some arguments I can agree with which are regular responded.

1. The current fiat system consumes much more energy. Besides the power consumption for a payment-processing systems you also need to consider that the current system consumes energy for building banks etc.

2. bitcoin mining takes place in regions with low energy costs (most likely equivalent to a power surplus) which prevents the need of energy transportation/conversion which would lead to a percentage of energy being wasted.

3. The generated heat from the mining can be used which would make the energy not totally wasted and benefits the one using the mining equipment as heater because of the mining reward. Example:http://www.cbc.ca/news/canada/manitoba/bitcoin-mining-greenhouse-fish-farm-1.4470295