Mode S
With the Eurocontrol requirement for the enhanced mode S, and the vast number of aircraft that are / will be exempt will there be enough aircraft with the "mode S display of alt preset" to have any benefit.
As to the what shall we preset, on a SID, I can see no reason (on Gulfstreams) not to set the CLEARED ALT, after all this is as well as an instruction that the A/P must respect, a memory aid, (CRM etc, etc).
On a FMS STAR, then unless the next alt or the bottom ALT is preset, the FMS will drop out of "blue" FMS V descent.
Lastly, what else does the enhanced Mode S display to ATC.....my mind boggles....
Glf