grdgradient - Hillshade from several angles
1) The new -Em option in grdgradient is not documented on the grdgradient man page.
From the cookbook:
"grdgradient now has -Em that gives results close to ESRI’s "hillshade" (but faster)."
2)Being able to make a multi-directional hillshade can make a map easy to read and very comfortable to look at. See https://blogs.esri.com/esri/arcgis/2014/07/14/introducing-esris-next-generation-hillshade/. I also posted about this in the forum some time ago; http://gmt.soest.hawaii.edu/boards/1/topics/1041. This would be a great feature.
That option probably never got documented because AFAIK (I did already some long time ago) it worked well in some cases but others were requiring a scaling factor for the shading effect do it's job.
Anyway, it was not that multiple directions shading that you pointed us to.