Again no need to sorry - it's very useful info, that you provided. I'm sorry that i provided wrong info -strange thing, that all USB 2.0 HUBS i used doesn't separate power from upstream port, i was really sure that it's common habit.

If they tend to separate it, they do it with some rationale, do they? What is the danger of using non-separating USB hub?

I encountered only one strange - amusing, but not dangerous - thing when using such a hub. It was powered by ATX power supply, that i use also to power my other things around computer, like WC (water cooling, not toilet) fan controller - using 5V to operate and 12V to power up ventilators. One time, my ATX was turned off, while HUB still connected to it and to my notebook. What was my surprise, when on notebook power-on, Fan controller also waked up - fans we're not turning, but controller itself worked. It turned out that power from my notebook flow by the upstream port of HUB and its power plug to the (powered down) ATX power supply, then, by its lines, to fan controller. I just wonder if this single-diode design can also separate-to-powerplug power, not only powerplug-to-separate - will check that.