Why is ReaderWriterLockSlim missing from Enterprise Library 4?

With the release of Enterprise Library 4 I thought for sure the Caching Application Block's use of the simple System.Threading.Monitor would be replaced with the new ReaderWriterLockSlim that is part of .Net 3.5. After all, Jeffrey Richter was right about the slow performing ReaderWriterLock, and now there is a better solution than what was in Enterprise Library 3.1, right? Yet in the source of EL 4 you can still find the calls to Monitor and some simple lock(object) statements. It seems like an easy performance win to me, but hopefully there is a good reason it's not included that I just don't know about.