Plus, what if something goes wrong with the $50 dollar compressor!?. You could get a leaky hose connecting the compressor to the machine. And the compressor would have to constantly be filling up the tank attached to the compressor (if your low end compressor even CAME with a tank, in which case it would just be on all the time), which makes it very noisy. And I know that when I'm working with pneumatic tools, the air compressor kicking on sometimes makes me jump. Wouldn't want to jump while you are stringing.
Overall, a good design and idea, but not practical enough to be better than a CP electric. I guess if you were well - versed in pneumatics, and had a lot of money to spend on a nice air compressor and to replace fittings and seals and whatnot regularly, it wouldn't be that bad. But why not just get a nice electric?
2x AG200 13.6 oz Blue Gear/Isospeed Baseline Spin
1x MW200g, 1x Mfil200g