Introduction:
Memory protection is a mechanism implemented in computer systems to prevent unauthorized access or modifications to memory locations. It ensures that each process running on the system can only access the memory assigned to it and cannot interfere with the memory of other processes. However, in certain types of systems, memory protection may not be necessary or useful.

Explanation:
Single user system:
In a single user system, there is only one user who has complete control over the system. This means that the user has full access to all the resources of the system, including memory. Since there is no need to protect memory from the user itself, memory protection is of no use in a single user system.

Non-multiprogramming system:
In a non-multiprogramming system, only one program is executed at a time. The entire system resources, including memory, are dedicated to that single program. In such systems, there is no need to protect memory from other programs or processes because there are no other programs running concurrently. Therefore, memory protection is not required in a non-multiprogramming system.

Non-multitasking system:
In a non-multitasking system, only one task or process is executed at a time. The system resources, including memory, are dedicated to that single task. Similar to a non-multiprogramming system, there is no need to protect memory from other tasks or processes because there are no other tasks running concurrently. Hence, memory protection is not necessary in a non-multitasking system.

Conclusion:
In summary, memory protection is not useful in a single user system, non-multiprogramming system, or non-multitasking system. These types of systems do not have multiple users, multiple programs, or multiple tasks running concurrently, eliminating the need for memory protection. However, in multi-user, multiprogramming, or multitasking systems where multiple processes or tasks run simultaneously, memory protection becomes crucial to ensure the stability, security, and isolation of each process.