Alexander Johannesen's essay "Here is a How to Topic Maps, Sir!":
"The truth about relational databases is that they really are Topic Maps that are trying to get out. Think about what your RDBMS is trying to do; you have a lot of tables with information bits, and you create relations between them to represent something vital to your business requirements, write SQL to mirror that and try your best at fixing a user interface on top to make it all work. The more relations you've got, the more complex your model is going to be. And for what? To create an application that that both a computer and human can handle well.
Where do you stop expanding your model and when? When it gets too complex? Too slow? Too unmaintainable? Too crazy to keep going? Too often you get bogged down in the design of models; what relations are hogs, which ones are necessary, which ones are not?"