There are a few projects attempting to write JIT compilers and even an entire JVM in Java itself. All such projects require much more reflective power than the simple introspection of the Reflection API. Some projects currently in development include OpenJIT, a open source JIT compiler written almost entirely in Java using reflection techniques (Japan), the MetaXa group which is working on an alternative approach to Java reflection and Jalapeno, an IBM effort to build the entire JVM and JIT in Java using a self-descriptive Java framework. (See OpenJIT paper, ECOOP 2000 for references.)
For these types of projects, reflection is essential. However, currently they are either using native code implementations or modifing the JVM itself to be able to do what they want. See below for more.