This is a good thread…not sure how I missed it the first time around.
To me, a rear center mesh pocket:
1) Allows me to fully pack my backpack from the protected confines of my shelter, and then once everything is packed I can jump out on those rainy mornings and stuff the shelter into the mesh center pocket (or a side pocket perhaps). It's not about drying…just about being able to close up my pack under the protection of my shelter. In reverse, it allows me to set up my shelter quicker because I don't have to open my pack. With that said, I only employ this technique in difficult weather conditions. On regular days I prefer to keep my shelter weight inside the pack for a better center of gravity.
2) Handy Access – I could use hipbelt pockets, but Zpacks center pocket is only 0.3oz more than two hipbelt pockets and you get a lot more room which makes it useful for stuffing clothing layers. During most days, the rear center pocket is place for my snacks and for adjusting my clothing layering (ie. windshirt).
In a different note, I just noticed that Zpacks is offering their Zero packs in the new Cuben/Nylon hybrid (2.9oz) material. I'm not sure this is needed, but it is neat.

