Using the parent <-> child relationship to create links between items has a lot of advantages :
1- you can directly see the items when they're linked (below the parent, or using the context parent, or just looking at the properties pane)
2- all parents or children are visible in the properties pane,
3- you can transfer data from one item to the other through inheritance or column equations,
etc.
However, some of these strengths are also weaknesses. One of them : if an item has multiple parents, which themselves are children of multiple parents, etc. column equations can rapidly spiral out of control and IQ's performance decreases dramatically. This is a big problem in my DB now. Sometimes an operation on 1 item takes 15s !