What advantages this has over an Arduino powered board ?
The Arduino is an 8-bit micro-controller running at 16 to 20 MHz. The BeagleBone hooked to a BeBoPr board is running 700-1000 MHz with hard floating point, and includes dual 32-bit 200 MHz micro-controllers for real-time tasks. Basically, using the BeagleBone and BeBoPr removes compute limits from what you can implement on the typical Arduino setup. LinuxCNC also has a very powerful and flexible real-time HAL layer, where it is possible to implement closed-loop servo control by just hooking up components…no coding or re-flashing firmware. And Linux runs on the same platform as the motion control, so you can leverage the entire Linux eco-system (for instance, my A/D temperature reading code is written in Python, because it doesn’t need to run particularly fast).
Does $120 buy you the motor drivers too?
I haven’t bought a board from Circutco/Boardzoo, so you’d have to check with them, but I’m pretty sure it’s “bring your own Pololus”. 
There are some good deals online…the versions I’m using to print with right now came from RepRapDiscount (as part of a RAMPS kit), straight from Hong Kong via eBay.