For programmes involving shallow galactic surveys over large areas of sky, Parallel Mode offers a major efficiency gain or major additional scientific data with respect two separate PACS and SPIRE coverages. For other mapping programmes like deep galactic surveys or shallow extragalactic surveys, there may be some advantages with careful design of mosaicking, although not so great.
For very large shallow programmes, where the requested coverage is typically in the order of 100 square degrees or more, the observer could save a significant time compared to using the two instruments separately and covering the survey area twice. For these programmes the PACS observations to be obtained without compromising the sky coverage. This is the most likely instance in which Parallel Mode could be beneficial.
Deep surveys of nearby molecular clouds mapped with SPIRE could have a benefit of PM mode. Instead of doing SPIRE observations only, it could be conceivable that PACS is operated in parallel without compromising SPIRE data quality, providing a shallow PACS survey of the complete regions. This would effectively provide additional data for the same observing time. The benefits in terms of mapping efficiency and additional data would not be as great as for a larger shallow survey, but PACS data could be worthwhile nonetheless.
One could assume the survey area of deep extragalactic mapping programmes is considerable smaller than for galactic surveys. For such a programme the Parallel Mode gain on mapping efficiency is not a great benefit. These observations will cover a range of depths where both PACS and SPIRE sensitivities are demanding. As with the galactic surveys, it is possible that PACS could be operated in parallel to provide additional data but in considerable lower sensitivities.