I used to feel that the private inheritence is useless. Although we can implement the has-a semantics with private inheritence, it provides little benefits compared with object composition. Besides, in order to expose the privately inherited members to public, C++ introduced an awkward syntax, i.e. the using directives. These are the reasons why I didn't like the private inheritence at all.

Recently, I changed my mind. Since we only have to specify the function names in the using directives, all of the overloaded functions can be exported with one directive. As a result, we can reduce the numbers of helper functions. For example:

#include <iostream>

class A {
public:
  void test() { }
  void test() const { }
  void test(int i) { }
  void test(int i) const { }
  void test(int i, int j) { }
  void test(int i, int j) const  { }
};

class B : private A {
public:
  using A::test;
};

int main() {
  // All of these should work.

  B b;
  b.test();
  b.test(1);
  b.test(1, 2);

  const B &rb = b;
  rb.test();
  rb.test(1);
  rb.test(1, 2);
}

However, IMHO, private inheritence is still a good indication of bad class hierarchy deisgn.