Hi,
The modelling of structure will altogether change the support loading.
If you see the default rigid restraint stiffness used by CAESAR is 1.0E12 N/mm..CAESAR uses this stiffnesses for Y,X,Z and anchor supports.
If you ask a structural engineer to calculate the stifness of the pipe support , i am sure stiffness he will provide will be quite low.
In one of my projects we have used the actual support stiffness(provided by structural department)and i observed a drasctic change in support loads especailly anchor and limit stops.
Why it is not widely used is just matter of your client otherwise technically it is more realistic to take into account the supporting structure stiffness for analysis.