ORIGINAL: Lazer
The main reason is to keep the vibration level to a lower point and try and not place the whole airframe under a high frequency vibration affecting servos, RX, connectors, etc.[8D]
I would have put the main reason as noise reduction. Reduced airframe noise to be exact. Positive side effects are longer servo and electronics life, reduced RX pack power consumption & airframe life would also be extended.
Cheers,
Jason.