Hello, you are right, when the rode is taught the sentinel doesn''t add much (It always does add ''something'', but compared to the horizontal pull of a boat, then its just a drop in the bucket). Like I mentioned somewhere before, when I was anchored in New York Harbor July 3rd, 2000 and the front came through. My sentinel wasn''t doing much and recall seeing it just about out of the water after I was letting out more rode. The rode at that time was as tight as a cable. But I like having nylon rode for its great stretch and absorption of forces when gusts or swells hit.
With a very slack rode, the sentinel can help if you tend to be inj a location where there aree tide changes (like the Hudson River with 3-4 knots current). In between tide changes the rode could go very slack and all over the place depending on the wind and some anchors don''t set/reset as well in those conditions. Or worse yet, you might get a wrap around your keel or rudder. A sentinal in these conditions can help keep the rode down and the anchor shaft to rode geometry as close to the bottom as possible.
But like everyone says, All chain is better for this.