You're starting from a false premise, which is that Boost is good. In reality, Boost is stuff that either couldn't make the cut to get into the C++ standard library, or is so new that nobody knows whether it's any good or not.
Boost is stuff that everyone wants to use but can't because they're forced to use an old or broken compiler that isn't capable of building real C++0x/C++2011 code. It's a staging ground. How else are you going to use std::bind or std::thread et. al. with visual c++ 2005?
Also, there are plenty of nice modules in boost that just don't belong in the standard library. ASIO is quite nice, but it has io_service implementations that are platform dependent. Statechart is pretty good, but it isn't the only state machine implementation in Boost and neither is clearly better than the other. Spirit has it's uses, but does it belong in the standard library?
You would never catch me saying that Boost is good. Heaven forfend! Some of it is good, and other bits are awful. The Boost "style" is, however, quite popular in commercial C++ projects.
