I worked in aerospace, in engineering laboratories where we had to distribute 28volt DC supplies around large areas. A can confirm that we used a LOT of copper in these installations. Not too serious in what is already a high cost environment but wouldn't be economic for domestic use.
LED lighting may make low voltage economic for lighting in fairly compact buildings but available lamps I've seen are mostly 12volts.
Regarding the 5volt mains socket+USB units, what is anyone's experience with reliability? I put one in about two years ago and I'm going to have to replace it as the USB sockets are worn to the point of being intermittent.