Here is why you should explicitly handle all exceptions in a system that is designed to have a high level of availability:
A system that is intended to have a high level of availability means that the system has high requirement. The system should be accurate and should be available at all the time. So in such a system, errors and the wrong functioning of the system is intolerable. In order to develop such a high availability system, the exceptions are to be handled carefully. Even if an exception occurs, the system must be able to recover by itself and ensure availability. So all the exceptions are to be explicitly handled.