From all the discussions above, two issues emerge:
1. Certificate issuing is governed by the terms of the contract of employment.
2. Whether there is a contractual obligation or not, it would be good practice to issue an experience certificate to all employees who leave the company, irrespective of their length of service. The second point of view appears to be the more preferred one.
What Should the Certificate Contain?
This leads to the question: what should this certificate contain? If it is bare statistics about the Name, Designation, Date of joining, Date of leaving, Salary last drawn, and reason for leaving, there are no difficulties. Perhaps there is an implied "right" for the employee to have such a certificate from the employer. But beyond this, if any mention has to be made about their performance in the organization, it is debatable. Can a leaving employee force the employer to issue him a certificate stating that his performance was 'outstanding' or 'very good'? Such encomiums are at the discretion of the employer only. No one can compel him to do so.