I found these rails on the doors, except the ones in the back under the windows useless (I also never saw anyone who build such thing on his car and if someone would found that usefull, it had sure been done already).
It might look like "Camel Trophy" but...
... it makes it more likeliy to damage the doors in narrow sitiuations, as the "extension" of the door comes more close to the walls, rocks, trees, etc. That's why rock and treesliders are used, to keep the side of the car away from obstacles. Hanging stuff onto the door while driving is the opposite.
... using it with a net (to put recovery gear in, for example) will just pick up everything you come by and will leed to more scratches and tree branches entangled there...
... using it in a camp...to mount a table for example....at least with my familiy on tour, these doors are opened and closed every 5 miutes. No way to mount somethuing there.
... using it with a solid load, like a can, can lead to something get trapped between can and door and damage, scratshes etc. again....
... dirt trap and humidity trap
Carrying more load on tracks (or maybe streets if legal) could be a point where it might be usefull, water tanks for example (I don't think fuel is not the best idea in case of someone crashing into the side of the car).
And, mountung these rails with minor practically needs special tools to create the door profile, rust protection, additional work für mountung, weight...etc. And the door hinges must proof to be able to handle additional load here.
Cheers
AWo
aeropress.com
Like you I am especially interested in having the track under the rear side windows. That, so far is where I see them being really useful. I can see the other rails being handy at a drive in cinema. Somewhere for the shake and 
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