When the answer is merged, there is no way for the OP to know whether there are any new answers to his question because he is not notified.
When an answer from the duplicate question is merged, the OP is notified of the new answers to his question thereby, allowing the OP to evaluate the new answers and marking the best answer accordingly (where necessary).
Case in point:
I didn't know I had merged answers to one of my questions until I accidentally stumbled across the question due to notification of a new answer earlier yesterday. I don't ever remember being notified of the additional answers.