Expose is very nice but it doesn't <necessarily> obviate the need for tiling. For example, if you're using a 20"+ display and you'd like to tile two browser windows side by side, Expose isn't going to help you do it and, as far as I know, you're forced to do it manually. Given Expose's powerful windowing management features, it would seem trivial to add in a tiling feature.
It wouls be trivial for sure. FWIW, some apps on OS X do in fact have tiling and cascading... macromedia fireworks for example, and some others. It would be nice if the OS did for every app though.