Agreed, I wish the BBB had power and ground bus for the digital pins. If you've got a serial port, you can use the P3 serial adapter for only $5. Its easy to solder together. If I can do it, you can do it for sure. You'll be locked into their bootloader if you don't have an ISP programmer though.
It has the following benefits that swayed me in the direction of building these for my project.
1) Small size. A bit smaller even than the $50 robot PCB. Development boards like the 500 are quite large.
2) Breadboard compatible
3) Arduino libraries make it a snap to get started programming. Can use the lower level WinAVR libraries or even C++ if you wish.
4) Come disassembled, so I can place the headers where I want in the direction I want with the gender I want.
5) Not so much soldering.
6) Cheap when ordered unassembled, leaving room in the budget for other things like the Sharp sensor.
I'll be building shields in the next few weeks for mine so I can get the servos hooked up.