The argument typically boils down to environmental impact of the power draw, a great argument when the only perceived gain is a piece of digital currency that "doesn't really exist or have value".
That is only a partial truth. Yes, bitcoin miners draw power, probably made in a horrible coal driven power plant somewhere. Yes, their output is digital currency. But, compare the environmental impact of consuming electricity for mining versus the environmental/cost impact of a non-digital or centralized currency. The value in mining also comes from the benefit it provides to others. The coins you get for mining are a reward, not the actual job. The job miners are actually doing is ensuring the security of the network. That's something you usually pay banks to do, except you have to blindly trust them that they aren't letting anything bad go on. We see how that's turned out...
I think if you even just look at the innovation that's taken place over the past 5 years in the digital currency space, Bitcoin has had a much larger impact than just generating currency.
For example, one of the reasons bitcoin gets harder every X iterations is because they knew hardware would become more capable as time passed. It's one of the failsafes built into the system. Once hardware started to push ahead of the curve, bitcoin difficulty started increasing at a faster rate to compensate (because we were all mining faster). It self corrected, for lack of a better term.
In the end, it's an experiment. Your participation is purely opt-in and bitcoin itself may crash a horrible death. What it has done is open the door for a new type of currency, more suited for the future we are trying to build.