They are a rubber component, they wear. They are designed for use in an exhaust with a flexi, hence the FRP had an uprated item. Excessive engine movement will put undue stress on the manifold flange. This is all just simple physics. And no, I didn't have a Protek, I had a Milltek, but the design is the same on all of them, as all are just s/s copies of the original FRP manifold, of varying quality.
Now I have a different manifold, I have a Milltek EVO, which is sleeved and has a flexi section, meaning that engine movement is less of an issue, but I chose to replace the torque restrictor anyway as it was worn and allowing excess movement. This was on an 8 yr old car which had done around 75k miles.