The Photoshop CS2 Help Desk Book

Layer comps are included in my nominations for Most Flexible Method because you can take advantage of them to quickly change between various combinations of layers .

I suppose you could argue that layer comps should really be on the nominees list for "Photoshop techniques that can save you from making a whole bunch of separate documents," but I include them on my list because they still offer a simple way to try different options, which is a kind of flexibility, isn't it?