But more practically: what minimum power supply does the iPad Air need? What USB hub do I need for my iMac to charge my iPad Air?
You're confusing two things. USB 1.0 & 2.0 DATA provides 0.5 amps and USB 3.0 DATA provides up to 0.9 amps. That is for normal data connections. They can still provide up to 2.4 amps from a charging device or the designated fast charge port of a data hub.Then there is the matter of the power supply. Your iPad Air came with a 20 watt adapter, but "USB-A charging maxes out at 12 watts (5 volts, 2.4 amps) for Apple devices." (The New York Times - Wirecutter - The Best USB Phone Charger).
That may be optimistic. Other sources say that USB 1.0 and 2.0 can provide at most 0.5 amps over a downstream port, and that USB 3.0 can provide at most 0.9 amps. If you're using a USB-A ( USB 3.0 ) hub, that might mean that the hub can only provide 4.5 watts; less than 1/4th of the power output of the standard iPad Air power adapter.
It might relate to buying cases first day, versus a couple months after the product is out and the case-maker quirks and errors are sorted out.
There’s no question that Apple has struggled to craft a cohesive, compelling narrative for the iPad. For a long time, there seemed to be a distinct lack of product vision. Everyone likes to speculate over what role Steve Jobs ultimately intended the iPad to have in people’s lives, but not only is that pointless, it’s also irrelevant. We don’t need Steve to tell us what the iPad is good for. We know what it’s good for, and we can easily imagine what it could be good for, if only Apple would set it free.