Here's the real difference, a big stepper motor requires even better control and the motor itself better be precise too due to the weight!
What do they use in inkjet printers? DC Motors! Because there's little weight to shuffle around and therefore more precise movements can be handled BUT do you have thermal issues, really?
A 3D printer needs to be a few orders of magnitude more precise to print precisely to 1000ths of an inch compared to a inkjet
Another big difference is also that the print head on a normal 2D printer moves at a fixed speed, that speed maybe mixed with an encoder, makes the print head fire a little drop of ink out at the precise moment. A 3D printer can't do that, it must extrude the plastic at the right rate all the time, and must be able to make rapid changes of the speed it is doing it at. With a dc motor you got higher acceleration and de-acceleration than a stepper motor, plus it is much harder to control, not saying that it can't be done.
Most of the first 3D printers, both reprap's and makerbot's were all using DC motors in the beginning for the extruder, and they all had just those problems. Today not many use them any longer (last time I checked we had 1 on the reprap irc channel, and he was in the process of changing that to a stepper motor too).
Huge cnc machines use dc servo.
No, they also more and more use linear motors, but there are still some servo motors to be found in them.
And that brings up a whole new problem too, just to wrap it all up, because one of the older cnc lathes we got where I work AGAIN got encoder problems. So suddenly the X axis can fall 0.05mm without the machine knowing, because the encoder somehow failed to register some impulses. To be able to track a DC motor and make it turn exactly as you want it to, you need to have an encoder on, to get a good reliable result, you need a good encoder, to get a good encoder you need to spend even more money and need more wires to get the readings back to the controller.
Such an encoder for a 5mm shaft could be one from Zapp, which would then be around £36 for the encoder only, and seems to be in a good an usable quality... And a stepper motor that could perform same task would be just over £13...