My understanding is that flying in clouds=visible moisture so according to Boeing, as long as you are flying in clouds and TAT is equal to or above -40 degrees Celsius, engine anti ice must be ON which is exactly what I do. THe reason why the FCOM says it should be switched on during descent if TAT is below -40 is because temperature increases as you descend and if you remain in clouds, the TAT will eventually get into the "anti-ice on range"