Uh...when you can only get Ethernet, and you only have one USB-Ethernet port and realize the doc/app/thingy you need is on a USB....Pretty straight forward in my opinion
Z
Not when you're using 21st Century technology called 802.11n! I have NEVER needed my USB>Ethernet dongle. I always have it with me just in case, but to date it's still in its original plastic wrap. And it's RARE that I have ever needed a USB thumb drive for anything (maybe 10 times since I switched to the MBA). With Internet access through MobileMe to my AEBS drive at home, network access at home to the drive and printer, and WiFi when at all offices and home access to files isn't as demanding as requiring me to use a USB port.
The whole point of the MBA is it's built to take advantage of everything wirelessly. Sure if you want to sit everywhere with an Ethernet cable, two USB cables, audio in, audio out, a FW drive, and power all plugged in, you go right ahead and buy your MBP. I don't want the MBA to be burdened with anything it doesn't need... and one X USB is plenty sufficient for an ultraportable... and when the MBA gets its hopeful "big" update in Early 2011 it will probably just be two LightPeak ports. That way one can plug in a display or an additional secondary device. I love the MBA's design how all ports are hidden except the small power port. The port tray hides the ports nicely as the vast majority of the time it stays closed on mine - except when I am at my desk at one of three offices where I have 24" LED ACDs ready for connectivity. It will be even better when we're all using Intel wireless display tech and don't have to worry about that.
The point is if one is thinking about ports, just buy a damned MBP and enjoy. The MBA is all about wireless connectivity for the ultramobile professional.