USB is important for a few reasons:
#1- ingesting photos. OR even better, tethered shooting. Cameras use USB ports for data transfer and control of the camera. The time to preview pictures is while you are taking them, not when you return to the office. Not saying you can't do this with other platforms, but you can do this with EVERY camera using Surface/USB.
#2- Printers. Sure, wireless printing is great...sometimes. Again, most every printer works with USB. I've been visiting other locations, need to print something and the IT guy rolls his eyes when he sees I need to print a document from a tablet. Once he sees Surface/USB it's like, "Oh..okay, no problem, plug in here." Nobody wants to install some stupid app to let me print.
#3- Wired networking. Again, wireless is great- but sometimes it is not an option. A wired network dongle has saved me a few times.
#4- odd peripherals. Just last Friday I had to provide a butt-ton of files to a lawyer for 'discovery'. They provided an external hard-drive (Aegis Padlock Drive). USB...sure, plug that sucker in and I'll give you all the files you need.
In a professional environment I don't always have control over what I need to connect to. USB has been the most ubiquitous port over the last 10 years. Not having it on your computer locks out out of a LOT of stuff.
My Surface Pro 2 runs Photoshop just fine. Admittedly I don't use it as my primary editing machine (screen size) but when I need it, it is there. Not sure why you say the Surface is underpowered, I would say it is 'run of the mill' in performance. In 2013/2014 that means, "It is an absolute breeze to do photo editing." Also, the fact that the Surface Pro is an awesome digitizer, with a pen, is icing on the cake.
I'm not saying the Surface is the be-all and end-all, but it has features that are fantastic. Most of the detractors obviously haven't used one in a professional environment...where an Android tablet or iPad just don't do what I need them to do. Including having a USB port.
**Someone felt that I was trolling. I'm not a troll, just a person who enjoys using their Surface Pro for work and entertainment. Also, my previous job had me purchasing a LOT of equipment (1,800 users) and I spent soooo much money on iPads it was amazing. Most of those were deemed totally useless once the novelty wore off. I'm just trying to steer people toward what I feel is a very good solution to the mobile computer question.