The SecondIterator adapts the underlying iterator, provided that it dereferences to an std::pair. The only adaptation it does, is to dereference to the second field of the pair instead. This is important to being able to realize good iterator interfaces.
The Boost library contains a generic transform iterator which can be passed a functor to obtain such an adaptation of the dereferenced value. However, it has a bug in it which makes it incapable to work with incomplete types. We could create a similar transform iterator ourselves, but that would be laborous because the implementation needs to be able to deduce the return type of an arbitrary functor. In C++03, this can be approximated (laborously) but not fully realized. In C++11, this can be fully realized by decltype, however, the current implementation of decltype in Visual Studio 2010 is incorrect. Therefore, for the time being we have chosen to create this very specific iterator adaptor instead and delay the implementation of more generic alternatives.