It's been too long since I built a PC so I'll take your word on the faster/cheaper argument, but greener? Can you really build a server box that uses less power than a 1-2A USB device?
Not sure, but I think the point was that its "greener" because you can swap out individual parts over time using the same case and ultimately producing less waste. You may also argue that individual components are easier to recycle.
If form factors don't change, reusing the mounting hw, PSUs and enclosures can be doable. Some web shops wait until boxes die entirely before replacement while IT shops lifecycle out all gear (usually everything but racks) in 4-6 years.
These hotdog USB sticks aren't upgradable at all and they're limited to the processing power of 5-10W.