I have a piping system which is about 20 m long with 4 supports (either rest type or rest+guide type; no axial stops) and the system is terminating with a flange (2 meters after the last support). Infact, this flange is an interface between my project and future project. We do not have any clue about the future routing. Hence, the standard practice is to model the stress analysis system till the interface flange. Keep CNODE anchor at that point and extract displacements at interface flange. These displacements will be an input to the future project to consider in their analysis.

However, my client came up with a comment to include Rz and Rx at interface flange along with C node. Can somebody advise the advantage of modelling the rotations at cnode?

I only guess that client assume that the pipe it will be properly supported so will not deflect on vertical position and will not be subject to torsion.
I can not see what brings to him but....

So as to (hopefully) not under-design the supports being designed now.

It's the analyst's job to ensure that both new and "old" piping (and structural analyst) to ensure both new and "old" supports are functional and to code. So it'll need to be validated again.

It's the hope of the end user that the supports and piping won't need to be modified later.