Shielding is done / required / utilized and even custom designed for a lot of different reasons, but, simplisticaly:
- to keep what is inside, inside the shield
- to keep what is outside from interfering what is inside
So now the question becomes...... What are you trying to control with the shield?
Using a crude water analogy... water may be on a pipe. Where do you want the water to go ?
Go back to the sanitation sewer, air dry, or go to a designated sump which makes it easier to get into the sewer line or a designated sump system, just contain it ( ie; tank to tank ) or even a combination of two or three of them.
So grounding of the shield could be subjective and only the person looking at the big picture for what was considered may know why
what is being done with the shield is appropriate. But generalizations do come about and are not necessarily always true.
From a non-electrical ....and may be all "wet", just some thoughts,
RICH