I can't think of any instances when I have used smart linking, because it ordinarily does not link correctly. How you link is critical to your report results, and you should read up on this and learn to understand it fully. If you do this incorrectly, your report results will be inaccurate.
In general, an equal join requires that there be matching records in both tables, so that if one or the other table contains non-matching records, they will not appear on the report. A left outer join can be used if you want to show all records from the left-hand table and only matching records from the right-hand table, but you cannot use selection criteria on the right-hand table or it will "undo" the left outer join. Those are the only two joins I've ever had to use, but it is important to understand them.
It also matters whether you have placed fields from each table on the report--if not, and if you have not "enforced" the join, the join will have no impact on the table that has no fields displayed.
-LB