What it stems from is the customer who neglected their vehicle, especially their transmission and when symptoms would arise they would request the fluid and filter changed. Subsequently it accelerated the failure and the customer would blame the garage, so garages became wise to this and started questioning, advising and documenting before simply performing the service. Generally speaking if there aren't any issues you should be fine changing the fluid and filter without adverse effects, prolonging the life and performance of the transmission. If a transmission begins slipping and you then change the fluid you essentially will increase the rate at which it will ultimately fail. The gunk may be helping aid in what friction remains in clutch packs that are near failure.