Like everything in engineering, having stepper motors and other DC motors have their place. It depends on what you want to do. IN building a robot, yes you can use steppers only. But there are some cases where it might be easier or better to use steppers (either functionally or the availability of motors that can do the job with the support equipment you have available).
A servo motor is just a motor with feedback. You can add feedback to a servo, stepper, or AC motor. They are two separate things.
Stepper motors are good
- when you don't feel like gearing down something (assuming you can find a strong enough stepper)
-you don't want to deal with the back/forwards motioninvolved in holding a DC motor in a fixed position. With a stepper you just move it to wherever you want and continuously apply power to the winding and it just sits there. No need to measure the position and correct it unless something is able to overcome the holding torque of the stepper.
-the motor has to travel slowly in the manners described above (steppers get the most torque when moving slowly)