There is a risk of using an existing test event, for all of the reasons you listed. To NOT go directly to the existing event requires:
1) running a search and finding an existing event that is both open [not ended] and available with permission to edit
2) creating a new event
Both of these scenarios add a lot of extra steps to the test which do not relate to the goal of editing a test. While it may be more ideal for the test data, it skews the focus of the test. My question is: which is more inline with our [Mozilla] style? Going directly to the created event and editing ,or finding/creating an event and then editing?