Leyendo el código de combase.cpp, encuentro lo siguiente:

/* We have to ensure that we DON'T use a max macro, since these will typically   */                    
/* lead to one of the parameters being evaluated twice.  Since we are worried    */                    
/* about concurrency, we can't afford to access the m_cRef twice since we can't  */                    
/* afford to run the risk that its value having changed between accesses.        */                    

    template<class T> inline static T ourmax( const T & a, const T & b )                                   
    {
        return a > b ? a : b;       
    } 

No entiendo por qué "max macro hace que uno de los parámetros se evalúe dos veces".