Lets put it this way, how much resistance do you think the entire arduino board is? (the one being powered from this source) 10, 100, 1000 times larger then 10ohms?
If we put that in a voltage divider (1000/1010)*5V = 4.95V. I think its safe to say that a 0.05V drop on that resistor is the largest drop you could expect, I would think its going to be far less.
However that's not the only possible drop on that resistor if you have 2 USB like my schematic, its also going to drop the difference between the 2 sources across it. This is where thevenin theorem comes in.
The 10 ohms is to both drop the voltage difference from 2 sources and not create current limiting in the event that your only plugged into 1 USB.
I already stated that I was aware of the current feedback but that level of current shouldn't cause any issues. (10-20mA)
The resistor is there as an energy absorber for the voltage difference. I see no way that oscillations could start; that only happens when 2 regulators are truly in parallel with no resistance matching.
I'm aware of germanium diodes for smaller voltage drops, but I want 5V not 4.3V or 4.7V.
Aside from the current feedback which I think we all know isn't enough to cause issues on the regulators rated for currents 100+ times larger. Especially on USB ports that have there own protection and very like have there own diodes built in.