This post seems a little old so you may have already made your decision. I was in the store shopping for a new pack when I saw some webbing and had the same thoughts. I think that weight and maneuverability would be great. you could also customize it to suit your specific needs very nicely.
But for me the webbing that they had in store just looked like it would be a pain in the ass to deal with. what they had just didn't suit my needs, I guess. it would have been kind of all over the place and after i bought all the pouches to go with it, it was just cheaper and easier to go with a rucksack. I also figured that it would be a little "to much". I had visions of a man waring a balaclava standing in the entrance of Walmart waving an AK-47 over his head, screaming "death to the infidels", and figured that I would probably be met with the same reaction if I walked into town fully decked out in webbing.
what you show here looks better then what I was looking at in store, though. I think that webbing would be easier to stow and hide in some bushes then a backpack would be. all you really need a pack for, is to carry your shit while your on the move, right. I think it really depends on what your needs are, and what you want to deal with.