I forgot about this thread. Yes, shock covers trap all kinds of shit inside the shocks. Moisture and dirt being the two biggest enemies. They also rub all the powdercoat off of the springs, causing the springs to rust, and eventually break. I see it all the time.
The key to keeping your shocks lasting long, is to keep them clean. After you're done riding, clean the shocks. While you're riding, the shocks have a special seal on the outside of the seal head called a "wiper". It's ONLY job is to keep shit out of the shock.
A good way to tell if something like those are complete bullshit, is go look at race vehicles or buggies, and see if they run them. If you don't see shock covers on race vehicles, don't run them.