Operating SPIRE and PACS together increases the load on the helium tank of Herschel by much less than a factor of two, because the total load is largely from the cryostat parasitics. Parallel operations is thus very favorable in terms of science per litre of helium, increasing the overall lifetime of the observatory.

Moreover, observations made in Parallel Mode will have more accurate relative PACS/SPIRE astrometry, because the relative angular offsets between the two arrays are fixed and accurately known and the pointing error will be the same for the two instruments throughout the observation.

The efficiency of scientific mission planning is also increased because the scheduling of PACS and SPIRE observations of the same sky area can be done within a single block. This reduces the total telescope slew time over the mission lifetime in favor of science time.