Forgive me for some selective editing with regard to the quotes.
To narrow the question a little more, what if all charts were in digital format running on some sort of display device (laptop or Avionics)......
At times there is no getting away from the fact that a paper document is prefferable. How many of us download a pdf file and print it for ease of use and to gain a better view of "the big picture" ?
How important would it be for this system to be 'full-up' available?
Absolutely vital that the desired information is readily accessable.
....... in an ETOPs configuration if they were digital and the unit subscribed to ship's power - a system that has probabilities of failure?
"Probabilities" is the magic word. The 767-300 aircraft that I fly has only 30 minutes battery power (assuming a 100% healthy battery). So, assuming your implied worse case scenario i.e. a total loss of A.C. power .... Well, I'm sure you've figured that one out already
If you think about it, it could be just as problematical in a non-ETOPs environment when your take-off or enroute alternate is more than 30 minutes away !!!!
Hope this helps.