In addition to what mciahel said above, to the best of my knowledge, Apple and a few manufacturers allow up to 1000 mA (1A) (possibly more) of current draw for high-power devices (such as to charge an iPad, in Apple's case), but as you can see, this would still not necessarily be enough for a USB-powered optical drive, which are already finicky with power supplies to begin with.
And for Apple's Mac devices, booting into Windows seems to drop the max current to the 500 mA standard. (Just connecting an iPad to my MBP while in Windows, for example, results in a "Not Charging" status, meaning it's getting under 1A of juice). The reason I remind you of this: the selection of software in OSX that handles Blu-ray is slightly smaller than the selection for Windows, and from time to time, you could possibly need to boot into Windows for whatever reasons. So getting the powered drive would be better for that.
ALSO, someone correct me if I'm wrong, but USB 2.0 already struggles to support 20x DVD writing (for me, at least). If you aren't careful, using a USB hub may present bandwidth issues.