This is very useful in organizing and browsing the code. Why is it not implemented? And if it would prevent some constructs why not have something similar for F#?
This is a feature that the F# team would love to add, but it is more of a "nice to have" sort of feature and isn't necessary to ship for v1 of the product release. We'll see more of these types of features make their way into F# in a future version of release of Visual Studio. For Visual Studio 2010, it's been a great deal of work to get F# to product quality.