[XorNot]

The problem is this doesn't tally well with the overall user experience.

I don't want my hard drive being littered with files owned by not-me, because they don't work properly when I need to rsync things and I can't change their ownership easily etc.

What I want is a kind of "sub-user": where I have sudo like powers over files owned by my user account by default, which are then dropped for individual commands - or something similar.

Which comes back to my original point: we've lots of mechanisms, but none of them actually wrap-well or seamlessly with how you actually work which makes them too much of a pain to use for the 99% of the time when everything is fine.