>The reasons you would choose to include a 'before-all' method will vary from case to case.

Not really. I would always purge anything that would cause tests to share state. I wouldn't do it on a case by case basis.

>What is it about your application code that would make you think that addUser is flaky enough to need a clean slate to run against?

The user already existing in the database? The behavior of the app would change in that case. Something has to wipe the db clean to test that scenario.

Thats why tests shouldn't share databases.