The answer is, of course, a matter of opinion. There is no objective way to say what the right policy is. My own view is that the answer depends on the sort of job that the employee is doing. All employees should be permitted some time for really important tasks. Employees whose jobs require more thought and more creativity should be allowed more leeway.
For the most part, employees should not use work computers for personal business. However, employers ought to give them some flexibility. If a worker has truly important things that need doing, the employer should allow it. This is because allowing employees to do this will improve their morale and their feeling that the employer is a good employer to work for.
Some employees should be given the right to use the computers for personal reasons in cases that are not terribly important. These are employees who have to do the most creative work. Workers of that sort should not be held to strict rules because you cannot compel people to be creative. Creative inspiration and general good ideas can come to people even when they are not actively doing what the employer might see as “work.”
For these reasons, it is a good idea for employers to give all employees some ability to use work computers for personal reasons and for them to give more leeway to workers whose jobs are inherently creative.