We operate the 100/200/300 series and only have such restrictions on the 100. My understanding is that with the yaw damper ON, reaction to a OEI is impeded as the damper will attempt to damp out the correcting yaw input by the pilot. If your BRW (actual or assumed) is greater than 14.4t, the V1 is greater that 91kts giving a little more directional authority in the event of a v1 go. It only appies to flap 5 take-off. All flap 15 take-offs are yaw damper off. Doesn't apply to the 200/300 though. There are so many mod status differences for the aircraft it could be different for different operators.