I struggled with the same damn question! Grimm, I'm incredulous that they always need to be replaced if the lower plenum is removed, but I don't have the experience to know whether not replacing them is going to cause a leak which would not only be difficult to diagnose but painful to fix. It's conspicuous too that the upper gaskets aren't nearly as expensive. Nissan definitely has figured out how to price their parts... much to our disadvantage.