As Ex FSO GRIFFO quoted, it can be used in the event that the engine driven fuel pump fails.
As far as I was concerned when I was flying the beast was that the aux pumps can be operated for a prolonged period of time in the above sitation without any issue.
But remember they are a backup for the engine driven pump and as such I wouldnt be inclined to fly multiple sectors with only the aux pump(s). If the engine driven fuel pump failed inflight I would return to departure aerodrome, divert or continue to the destination as I saw fit and once I landed I wouldn't be taking off again.
