Suppose you're developing an application with a rich backend, management tools and a thin web interface.
Luckily, you're working on one of the companies out there already integrating JDK 5.0, so your backend will be coded using generic, new syntax and everything you can dream about. However, BEA haven't certified their server for JDK 5.0 yet, so your web interface will have to stick to 1.4 features.
In other words: every module but the web module will use 5.0 language features. I can already select a different JDK to my web module (and this is great), but what about the language features?