Katharina, I just love your way of thinking. Every one of your posts makes me think or cock my head. It's delightful!
I think a lot of what you're talking about is cultural. Japanese when they go hiking together nearly always share everything. Their idea of privacy is different, too, and so they don't feel they have to hole up in a private shelter. I think that is actually how most of human existence has been. This new, "I'm a rugged individual" way of thinking is quite modern and probably wouldn't have worked in times past when people depended on one another for survival much more than they do (immediately visibly) today. If you read old accounts of people traveling in the wilderness, even in America, people shared a lot more space together than Americans do today. They were different back then. Personally one of the reasons I love the mountains so much is that they force you to take care of one another and to recognize the value of each of our lives, so sharing, to me, is only natural.
Thing is, when you only bring the barest minimum to get through it is difficult to share, unless you've planned everything together from the start. As a diabetic, for instance, I need to know precisely how much food I have and must carry an extra amount for emergencies, that I cannot share unless I am coming out of a walk and know that I will soon have food sources available. When I travel with others, though, even just friends, we will very often share a shelter, pot, stove, water bag.
The funny thing is it's the ultralight backpacking crowd that tend to be the most independent. Mountaineers, because of the necessity to keep things as light and simple as possible, will usually share quarters. I think this is another area that we as UL hikers could learn something about going even lighter... overcoming our personal zones of discomfort.
Interesting point is that Ray Jardin, with all his talk of each person carrying this certain amount of equipment almost never talked about solo set ups... all his accounts and advice is for a couple traveling together, with whom he shared almost everything. It makes you wonder what his set up would be like if he traveled alone. What would the weights be?