As you know Qlik's Associative Model provides easy association of tables by simply "associating" them using a common field name.
If you have a lot of dimensional data you probably don't want to have a table pointing to 20 other tables so that your model is cleaner and has fewer "hops." But should you use a plain JOIN or a LEFT JOIN.
This video utilizes the General Hospital data set (Click here to get it yourself) to demonstrate an example of a one to one relationship between tables that can be joined together. Narrative explains when a simple JOIN is the right way to go and when a LEFT JOIN should be used instead.