I think it's mainly for hyper milers who carry a tent and what not with them (and don't see a wall socket for a month).. However with a di2 battery easily lasting 2500 km, I would just bring another battery (or 2). They're only 50 grams
But I already have USB charging from my dynamo thanks to a D1. Just carry the Di2 charger which takes input from that USB power. I don't need a wall socket to charge my Di2 while riding. It makes no sense to me to wire this up permanently.
I think it's mainly for hyper milers who carry a tent and what not with them (and don't see a wall socket for a month).. However with a di2 battery easily lasting 2500 km, I would just bring another battery (or 2). They're only 50 grams