My feeling is that if the committee can't write clean, efficient, proper C++ code, how they hell are we supposed to be able to? If the committee can't write a simple vector implementation, then they need to get back to the drawing board and figure out why they cannot, rather than just pass the buck onto the vendors. I mean the whole reason Boost in its current form exists (and I love Boost) is simply because its the standard library that C++ needs.
Most of the people who work on compiler implementations and their corresponding STL implementations are actually part of the standard committee, and they actually work on the implementations of proposals while they are still in discussion to help nail down design issues.
The point of standards is that multiple people can independently implement them. A reference implementation would defeat the purpose.
If everyone used the same implementation, every quirk and defect of said implementation would de facto become part of the standard as people would end writing code unwittingly relying on them.