I've been told by many people in the know that it helps dapen vibrations in the truck, since its on the exaust i would think in nissans testing they found a weird vibration w/ the exaust and that stoped it!
It's also great for reducing vibrations from your garage floor, mine has been there for 2-3 years. Actually it sits right next to my stock exhaust and my Banks tailpipe. It's not welded on it bolts on unless they changed things since 05.
Yeah, I kept mine off when I installed my exhaust, but I wonder if it would lessen the sound at all if I put it back on. Sounds like an experiment for the summer.