There are even better fits for modern CPU architectures and hierarchical memory systems (but not on HDDs) than B-Tree based structures, like the adaptive radix tree: http://wwwkemper.informatik.tu-muenchen.de/~leis/papers/ART....

I think B-Trees are mostly important for educational purposes, since this is a very important general way of organizing data. For real world usage, there are hundreds of different structures optimized for different use cases.