STL smart pointers are all fun and games until you start using threads, at which point you realize that automagically flipping the (now atomic) counter back and forth behind the scenes every time the pointer can't be safely passed as a reference is a performance disaster.

Manually referencing and de-referencing exactly when needed avoids most of this. Sometimes it's perfectly safe to pass the same reference through several layers without changing the number of references.

As an added bonus YOU get do decide which references, if any, need to support multiple threads.