http://www.cncroutersource.com/stepper-vs-servo.html
Steppers are great for applying lots of force at low expectations of speed.
You can gear a stepper up to get more speed from the resulting output.
This works because the stepper pours torque in while moving slowly.
Other kinds of motors generally need to be geared down.
They produce power and speed in varying characteristics.
Most motors for CNC need to be geared down they move too fast.
Gear a motor down and the output torque increases.
Upside to a DC motor on CNC if you are cutting fast under power you can get there by gearing down.
Upside to a stepper on CNC if you cutting slowly you can move accurately easily and cheaper.
Gantry mills are typical for working over largish working areas.
A 4'x8' sheet of plywood for example.
How long would you like to wait for the stepper to do that job?
If the load is light like a plasma cutter that is one thing.
What if you are pushing around a 1/2+HP router with a big face mill on a deep cut?
A stepper will finish the job and you can even close the stepper loop with a sensor but is your CNC control smart enough to advance a missed step or microstep? Many are not. A detected error will just cause a stop.
On the other hand with a DC motor as noted the resolution of movement is never taken for granted.
A DC motor can take a heck of a beating as FIRST motors do and like FIRST motors they can be operated overloaded.
A DC motor will maintain the torque even as the rate of movement increases.
There is no magic 'right answer' to this.
My point stands: you have CIMS and a way to drive them.
An equivalent servo control and motor would be many hundreds of dollars per axis.
FIRST teams build gearing systems all the time.
Where as more hobby people with no tools, way to fabricate anything or idea how to make or program controls will undoubtedly start with steppers a large percentage of FIRST teams are not faced with those limits.