Almost four years ago was the first time I ever used Mambo (the predecessor to Joomla!). As I played around with my newfound toy, I thought the template chooser was one of the coolest modules available. I could just imagine the possibilities. Users could customize the look of my site for how they wanted to view it. I could install 100 templates and let the visitor choose which one he or she liked best.
However, over time I came to realize that the template chooser module was a relatively useless feature that was very difficult to manage. Sure, I could install 100 templates and give users a choice, but those 100 templates would likely have very different layouts and include different sets of module positions. For example, one template might put the "left" module position in the left column while another template would place it at the bottom of the layout while yet another template might not include the left module position at all. You can imagine the headaches this type of situation might cause.