Because of this:

bool operator == (const Address &a, const Address &b)
{
  NS_ASSERT (a.m_type == b.m_type || 
	     a.m_type == 0 || 
	     b.m_type == 0);
  NS_ASSERT (a.GetLength() == b.GetLength());  
  return memcmp (a.m_data, b.m_data, a.m_len) == 0;
}

If I try to compare addresses of different types, NS-3 aborts.  Why?  Addresses of different types should just compare not equal, nothing more.  Especially since there is no method to get the type of a generic Address...