we are in the same situation, drywallers said it is easier/cheaper just to tear it all out and put new drywall in than to try to refloat chunky damaged drywall.

Besides, if you've damaged the paper, it's may be problematic. Furhter, you may find it wasn't suitable underlayment otherwise (I found regular drywall, not even green board, in my old house when I tore out the shower).

I also think that since you're already doing it, it's nice to have a chance to let the area behind the walls air out.

It doesn't sound like you're talking about inside a wet area (shower), but in general you want a good solid substrate for all tile installation. If the drywall is in pristine condition you probably don't need to replace it, but if it's damaged, it would probably be better to replace it rather than risk your tile installation being difficult or not staying on the wall.