What is the digital dividend?
Digital compression systems now available for digital television systems allow the transmission of several (up to six, depending on the coding and modulation techniques) standard digital television channels of acceptable quality in the radio-frequency spectrum previously used by a single analogue channel. Typically, there are four or five terrestrial analogue services in a given region, so their digitization into a single digital television channel will considerably reduce the overall use of spectrum..
..The amount of spectrum in the VHF and UHF bands that is above that nominally required to accommodate existing analogue programmes, and that might be thus potentially freed up in the switchover from analogue to digital television, is defined as the digital dividend.
Refer also this ITU-R paper: Digital Dividend Insights for spectrum decisions [1.617 KB]
With regard to PMSE it should be noted:
In principle a Digital Dividend can be expected in all analogue transmission technologies that can be digitized.
The key argument might be: The higher compression of raw information can be so less spectrum is needed.
High Compression in PMSE is not possible in principle due to specifics of PMSE, Analogue PMSE has already analogue compression.
Digital compression would only provide marginally higher compression. Overhead for digital would have to be paid, costs even extra spectrum.