servos are simplicity. once a servo is modified, send a signal to it and you can get it to do what you want. not outside circuitry is needed. this is what makes them perfect for small, sensory based robots like the $50 robot.
If i build robots that are more complex i prefer to use geared motors. this is because they are generally more accurate and easier to use when working with encoders and more complicated tasks like mapping. when i say more accurate, modifying servos is not an exact task, no 2 servos will be the same and factors like heat and stress on the servos will throw out the "ideal" center position making them not accurate when in operation.
and another factor that leads people to servos for basic function is prices, servos are generally a lot cheaper than a geared motor that you can find.