Actually I do use a Booq Vyper sleeve for my MBP, so in the summer if I'm slumming about to several locations and know I won't need my tools then I just grab the sleeve and go. It weighs less than my coworker's waterfield messenger bag with his 13" MBA. One thing I wish the Vyper (and sleeves in general) had was a D-ring for attaching a lanyard or single point strap. It would leave my. hands free and I could attach the few items like charger, Ethernet dongle, and precision screwdrivers to the strap.
The charger generally ends up in my pocket, but I've gone without it too. This works well enough I've considered switching to the 15" MBP for the extra screen. Many times throughout the day I have remote support sessions going to end users with dual 1080p monitors.
I could get by with an Air and have considered switching to the 11" model this year, and keeping external monitors in key strategic locations. Still I have a feeling I will end up with a 13" or perhaps even a 15" Pro, and change out my bag to something more streamlined. I guess we will see when the new equipment starts showing up soon. I don't know, that's the future. I'm just giving present experience for reference.
I think your post is ore representative of what many photographers do. If your going to be doing photoshop work on larger HD files then your going to generally want more than 4GB of RAM which the MBA cannot provide. Also, while the MBA is light than the MBP as I have said its not as if the MBP weighs 100lbs, the MBP is plenty light in its own respect. Further the OP stated he was going to be taking hundreds perhaps thousands of pictures that end up being larger in size (storage size), which he would benefit from the 500GB HDD in the MBP, unless he wants to tote around an external HDD. I still think the OP is best off with a MBP. Don't get me wrong, I have a MBA too and I love it, its great when I am just running out the door and need to grab a computer. However when I have to do some actual work I always grab for the MBP.