True, the filters are an option as some/many don't have them installed when new. However, it's a very simple mod to add them as the door/cover where they go may need to be cut open.

That being said, it's a good idea to have them in there. I'll admit that we haven't had in-cabin air filters fairly recently, as far as automotive history goes. Cars and trucks have always just sucked in the outside air. And the X's do that too still. But, the difference is that with the advent and availability of the filters, especially in the X's, the ROUTING of that air has changed. The X's have dead air places and corners where, as TJ mentioned, leaves, dead bugs and even "live" stuff (think mold) can collect. Not good stuff to be glowing into your cabin and breathing!

So no, Nissan doesn't want you to die while driving your truck. But, the filters sure make a BIG difference in the quality of the air you're breathing.

Add 'em if you don't have 'em and keep 'em clean so they can do their job.