@JeenChew, @AjitKumar is correct, the AP will disable one spatial stream (the MR42 is 3x3, not 4x4, so it will disable one radio chain and go from 3x3 to 2x2). In addition it will disable its 3rd Air Marshal radio and its 4th Bluetooth Low Energy radio. So you lose BLE for proximity engagement type stuff. You DO still have Air Marshal and spectrum analysis functionality, but the AP has to do those functions opportunistically by going off-channel on the client serving radios, and will not be as robust as doing all that stuff on a dedicated dual-band radio.
Even though the AP will likely not draw more than 15 watts, or even 10 to 12 watts perhaps, during normal operation, it would seem it could run fine on 802.3af power. But it can actually peak above 18 watts, and so it must be able to negotiate 802.3at power at bootup in order to avoid running low power mode.