That's true if you supply a 5v servo with 12v. But not if you have a 12v servo.
True but you would be rather silly running a 12V servo off 5V, it might not even work. Where as a stepping motor can be run to advantage off any high voltage providing you keep the current down to the limits. Even a resistor to limit the current will give you an increase in the potential top speed.
It would want to be gold-plated at that price
Well it has got a processor embedded in it so it runs with the minimum of overhead from the Arduino. It also has RAM and EEPROM in it. But having said that it does loop pricy.