It's irrelevant, because I _do_ have root access to my machine,
How nice to be you. Since the argument is entirely about people who _don't_, your point it? It is clear that the only sensible default is no default. Someone else has said it recently and said it much better.
I think the right question is "how many people prefer user installs over system installs, wrt. their hackage packages?".
No, because the costs are asymmetric.
Endusers, of course, might have other preferences, but cabal doesn't (IMHO) cater to them, directly: It caters to distribution packages (or windows installers, or whatever), so cabal's default behaviour is quite irrelevant for those cases.
The clear impression I've received on this mailing list is that cabal is _also_ for people who are using Haskell and find that there's a new package reported in HWN that they'd like to have. If you are now telling me that I should ignore it because I'm not making distribution packages or Windows installers, fine, just tell me what to use instead.