The IAT sensor is integral to the MAP sensor. The electrical output of a pressure sensing element is temperature dependent so basically all pressure sensors have a temperature sensor built into them to compensate for temperature so they're reasonably accurate. The VW MAP sensor take advantage of this and uses the 4th pin to give temperature feedback.
My guess is that in an OEM tune it's for sure used.
In an aftermarket tune, maybe, maybe not - depends on the tuner
It's main purpose should be to trim fuel delivery so you have a consistent amount of smoke regardless of ambient temperature. if you're not too concerned about this, then you can just leave it in the corner like you describe. It will do some gross compensation, but if there's a lot more tuning that you're doing, then it's unlikely that you'll achieve the level of refinement where this will make/break anything - especially if you're using a MAF based tune.