The implementation of that b-tree is actually pretty difficult (I've attempted in the past, but it depends on your skill of course) and the constant factor is much larger. (It's somewhat difficult to implement because when you remove a node, tricky things happen and you need to keep the sub-tree sizes consistent). Granted, whatever floats your boat!
Sure B-trees are hard to write from the ground up but there are plenty of good implementations already the Internet that can be modified.
The extra functionality is a simple enough addition to any binary-tree-type data structure that I was annoyed that Boost/Qt didn't have it already included.
