Originally Posted by KenSt

Sensors are the norm and required in many industrial applications for brushless motors. They provide maximum torque at a stalled condition just like a brushed motor and won't switch to the next set of phases until the rotor gets there. Ideal for RC cars.

Sensorless works in RC planes because we don't care if the motor stutters SLIGHTLY at start up and the prop doesn't need maximum torque to start spinning.

Considering the encoder of a servo motor to be a superior form of "sensor", that is why servo motor is able to produce constant torque no matter what is the current speed. am i right?

Correct, the typical industrial servo has a flat speed torque curve until you get to the "knee" where back EMF drives up the current until you run out of DC Bus. Yes, an encoder is pretty handy in commutating a brushless motor. The best thing about that is for precise position control. Unlike a stepper, you can position and hold torque to a count on an encoder.
Curtis