Join date is relevant when I thought my user-pref to default watch key categories when they sign up.
As it is, there is no Watch a category - that entire phrase is a misconception*
There is only Automatically watch new topics in this category. And this is not what I need or expect.
EDIT - This was addressed in
*It states this clear enough in user preferences.
You will automatically watch all new topics in these categories. You will be notified of all new posts and topics, and a count of new posts will also appear next to the topic.