We're seeing more brushless motor for AEG use and Krampus Airsoft takes a a look at one from Warhead Industries... "The constant contact of the brushes with the power terminals of a brushed motor is necessary for it to maintain its rotation. However, the friction that this creates means that a huge portion of the energy generated by the rotational motion is wasted as heat. Generally, the efficiency of brushed motors is limited within the range of 75% to 80%.

Brushed motors are notorious for having parts that get worn out after a while, requiring frequent dismantling and parts replacement. This is because the brushes of the commutator need to maintain almost constant contact with the terminals of the power source, even as the commutator rotates rapidly. When brushes get worn out and can no longer maintain contact, a brushed DC motor basically becomes useless.
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This is less of a problem with brushless motors. In fact, there is essentially no contact between the rotor and stator of a brushless motor, so wear and tear are greatly reduced."