Art's Cyclery had this to say:
When it comes to carbon steerers there is little need for the additional friction that carbon paste provides, as it is very rare that anyone encounters a situation on a road bike where your front wheel is being pushed hard in a direction opposite to the direction you are turning the handlebars. The exception is during a crash. In this situation it’s better to have the stem slip on the steerer to lessen the amount of force transmitted to the steerer tube.
The other concern with applying carbon paste to steerer tubes is that the paste could make its way into the headset where it could grind down moving parts and possibly wear the steerer where the compression split ring interfaces with the steerer.