Paul has basically summed up the pragmatism beats purity side of the argument. Whether we like it or not, users are mostly coming to python.org for their installer, and we need to support that.

That said, we can do some things to support all three cases once we acknowledge that they exist and identify who they're for. For example, an all users install could become an advanced option, which will move most users into per-user installs. We could also release a plain zip file of the installed layout and clearly mark it as being for app bundles and not a portable install for users. There are changes to getpath and the registry lookup that will better serve non-system installs, though we really need PEP 432 for that to be feasible.

Some bigger changes I wouldn't mind would be renaming python.exe to python35.exe and then py.exe to python.exe, which would let users have a system Python that interacts well with registered per-user versions and shebang lines. These largely depend on whether we've got the stomach to be so dramatic :) With 3.5 beta coming up so soon, we may be better to avoid rocking the boat now and look towards 3.6, but maybe this is more urgent than that?