Saying that LibreOffice has not improved their Microsoft Office document compatibility in the 4.0 release is inaccurate. They HAVE improved it. They DID add a couple of formats that they did NOT previously even support (Publisher and Visio). They have also greatly improved formula stuff too.
Now that isn't to say that the import is perfect... or that it will ever be perfect... mainly because as previously stated, they use somewhat open but not totally open formats. Sure Microsoft went through the ISO standards body and got their OOXML format ratified... but just how they did that is a somewhat scandalous story... and the fact that the container is an "open standard" doesn't mean it isn't full of undocumented binary blobs. That is all by design.
Some pragmatic people might want to reward Microsoft for gaining and maintaining a document format monopoly. I prefer not to whenever possible.