I don't know why, it just doesn't seem worth it. All that extra bulk on the back (removing the ability to carry a pack) for 1200+ paintballs? Let's say the 1200+ translates to 1300 balls...
1300 balls minus the 200 you start the game in your loader with means there's only 1100 paintballs extra. If the average pod has 140 paintballs in it, then you are looking at just over 7 pods of paint (7.85 pods). Great for not reloading, but now you got all that added bulk for only 7 pods. You would have to have someone there just to dump paint for the heavy gunner into the bin instead of having it where the gunner can dump it himself into his loader.
Sure it can feed 30 bps, but even a maxed out Ego can't shoot that fast so there really is no point in that. Plus, where are you going to put the 180 ci of air?
I like the thinking, but give me Carl's pack any day of the week for my heavy gunning needs.