monore wrote:The keyword here is "Recommend". Why is this recommended?
Older versions of USB forbid downstream devices from sending power back to the hub. USB-OTG changed that by letting devices be both USB hubs and USB devices.
Because of that, USB-OTG devices don't have any isolation between their own supply and their USB ports.
It's generally a bad idea to put power supplies in parallel. The voltages are never exactly the same, and the one with higher voltage will try to push current backwards through the one with lower voltage.
That's a problem because all power supplies can be modeled as a perfect voltage source in series with a resistor. The resistor represents the power supply's tendency to lose voltage when the current load increases. We want power supplies to maintain voltage across a wide range of loads, so we want the series resistance to be as low as possible.
Putting two power sources in parallel ends up being equivalent to putting two low-value resistors between two different voltages. Even a small voltage mismatch allows a large amount of current to flow. Most power supplies aren't designed to handle back-power, and will probably be damaged by it.
The OTG USB hub is mostly used with the RasPi Zero, which is an exercise in designing down to the lowest price point. Its power system doesn't have things like protection from back-power coming through the USB hub while the Zero gets power from its micro-B jack.
That's a recipe for people killing Zeros as fast as they can plug the hubs in, which we don't recommend.. or refund or replace.