The Cardas caps were among a group of tweaks reviewed not long ago in "The Sensible Sound" (TSS). Now, I realize that TSS is not the ultimate high-end reviewer, but several of their writers have good systems, and like the rest of us, they have two ears and the ability to judge effects of tweaks on their system. The TSS article pretty much concluded that the Cardas caps added nothing (at best) and maybe even degraded the sound a bit.
Personally, I think that if you are concerned about dust getting into the inside of a component, then a plastic cap will work fine, and it's a whole lot cheaper than the Cardas gizmos. If your concern is about shunts vs. shields, then Sean has already given the best response above, and there is no need for me to comment further. (Good job, as always, Sean.)