I agree fully that, while there may well be bad power supply/bad hub issues for a number of users, there are a growing number of examples where the high power/low power usb device mix is the most likely cause of pi user problems, my own included.
I'm seeing almost exactly the same issue as Marsman2020 and it is my biggest headache using the Pi. I for one really appreciate that he has produced some top notch diagnostic evidence on Github #29 to support his conclusions and remove the hub/power supply from the equation.
For my part I have a no-name 7 port powered hub with decent quality supply, I am connecting a wired kb, wired mouse and a 16GB USB pen drive. I have wired Ethernet.
I have taken measurements on the board between TP1 and TP2 with a multi-meter as well as checking the stability generally of my power supplies.
Firstly the connections used and the issue I see:
Kb, mouse, pendrive connected to powered hub, Pi powered from hub, the usb uplink wire has had the 'cut the red wire' treatment, so is data only.
In console, I can see the pendrive, the kb works and I get reasonable internet connection.
Start LXDE and the network connection drops, then occasionally comes back up. The pendrive volume will also drop off, then reappear as if inserted. Most of the time I have neither the 16GB volume nor a working net connection.
So far I have tried Logitech mouse, Fujitsu Mouse and even an old wireless mouse from Dabs own range. Same issues seen each time (actually one of the mice didn't seem to want to work much at all).
I have tried an FJ Siemens keyboard and a Bakker S-board (portable USB kb). Same problem manifests again after starting LXDE. I borrowed a kb with two USB ports built in but this seemed to cause kernel panic when all connected with the powered hub (effectively resulting in three hubs being present).
Power supplies, I started with Kindle PS 700ma to Pi and no hub. All stable. TP1-TP2 remained between 4.93v and 4.90v.
Added hub on a working 1A supply, put kb and mouse on it, powered Pi from Kindle PS, uplink from hub to Pi (no red wire). All OK, LXDE/Midori OK. Multimeter had similar ratings to last, occasional drop by up to 0.05v.
Put the pendrive in hub, then restarted, All seemed OK but no network after starting LXDE, restarted and checked network back in console (it was), then start LXDE and it's gone again. Pendrive seemed to remain at this stage.
Swapped to a known (and stable) 2A supply for hub & changed to power Pi from hub (removing Kindle PS). Same issue each time, without X all seems to work, add in LXDE and no network. Also, at about this time the pendrive seemed to corrupt, it appeared then disappeared, then appeared then I saw an I/O error dialogue and hasn't been mounting since.
My personal thoughts (from a rank amateur) - the mouse drivers aren't needed in the console, so they aren't loaded - USB/network combo works. The kb and the pendrive (low/high power?) seem to work together. When X starts that causes the mouse drivers to kick in, conflicting with the network drivers actually causing USB instabilities across the whole Pi. The mouse/network combination seems to be most toxic, but have the ability to then kill any other attached USB device - particularly one that is high powered.
I vote for kernel/driver instability being the root cause and one needing urgent attention by the Foundation and some of these eminent and excellent supporters we have on the site. Within my limited expertise I will assist by testing any new build or patch that becomes available.
Never trust a man, who when left alone with a tea cosey, doesn't try it on.