I'm proposing we should have a more sensible default for database location, either in a globally writable space or a user's home directory.

Making a folder and chowning it (as per our directions) is not any different than simply using a subdirectory of the home directory (except it requires zero steps instead of two).

Security: same.
Support: better (no steps vs. two in most cases) and everyone knows and has access to $HOME no matter what platform you are on.
Documentation: your data lives in $HOME/mongodb.

I think that this change is worth the effort. New users will be happy that they can now start using Mongo immediately without configuration.