Anyone know what this vent is for? I want to make some diamond plate corners so I can leave the plastic corners off without looking like crap. I can't figure out why the vent is on there and I need to know if it is worth special effort to accomidate it.

Cross ventilation of the cabin.

I imagine it helps keep your windows from blowing out when the X is parked in the hot sun with the windows up; among other things.

Seems like a huge vent for cross venting the cabin. I think I could accomplish that with a few vent holes, possibly even just pulling out the rubber drain plugs.

???

Also, you've got to figure when you slam your doors or rear hatch, it forces a lot of air into your vehicle, and it's got to go somewhere.

I remember the last time I got my windshield replaced in my old car that they told me to lower the windows before closing the doors for the first 2 days so it doesn't unseat the new windshielf before the sealant secures it.

Ya, that vent is there to allow excessive air pressure out of the car when you shut the doors. Yes, you could accomplish the same thing with some holes, on unplugging some of the plugs, but this would allow water into your interior where it can hang out in the cracks and make a nice little cozy rust cavern.