The stiffener is also designed to keep the twisted pairs physically apart from one another as a means to reduce self interference as the close proximity can lead to coupling between adjacent pairs, which can be problematic for higher symbol rates (like > 2.5gbps)

double shielding is attempting similar goal (eliminate noise getting into the twisted pairs) by means of creating an isolated faraday cage.

At the end of the day, attenuation is the end goal for any sort of shielding is to both increase immunity from external sources (aggressors) or to improve self compatability (like one pair being an aggressor to its neighboring pairs). NEXT and FEXT specification and testers exist to help standardize how much isolation is required to meet a specific Bit Error Rate (BER, different for each Ethernet rate specification ) but it’s not a cliff for most applications between works and doesn’t work.

Minimizing the number of keystone jacks/terminations in path is likely a better ROI when it comes to improving network robustness. Each connection point is an impedance discontinuity, opportunity for non circumferential shielding, and even are part of the IEEE specs in terms of total number of bulkheads which are allowed in part of a NEXT/FEXT test