I believe air pumps are usually fed off the turret in the cab, so it can be turned of and on from inside, or fed from the auxiliary turret in front of the cab with a valve handle extended inside. My point being that with the source of steam so far from the smokebox front or pilot deck, there is a lot more potential for large amounts of condensation in the steam line by the time it reaches the pump. This wouldn't be a problem going down grade with the pump running often, but if the engine is standing in a yard, or running with infrequent use of the brakes the air pump it seems to me like could get some pretty big chunks of wet steam.
I think that on some of the large superpower with front end throttles, the air pump receives superheated steam, which would presumably come from the superheater header ahead of the throttle in the smokebox, making the pilot deck a much more advantageous place to mount the air pump.