Wouldn't it be a much bigger pain to refill an air tank? Even if you have your own compressor, I can't see how that would be any easier.
I have to say, I've always been pretty skeptical of adding power to a bike, too, but I rode a Stokemonkey-equipped cargo bike a couple weeks ago and it all really started to make sense -- just that nice little boost getting started at a light, or a mild assist going up a hill with a heavy load. It almost felt like an alternative to shifting -- just stay in a nice, reasonably still gear, and add a bit of power until you get up to speed. I'm still a big fan of actually turning one's own pedals, but that experience has me thinking...