Question

why should a circular saw that does not have an upper blade guard never be used?
the trigger lock will not work properly.
the exposed blade poses a danger.
the safety features will be disabled.

Explanation:

A circular saw's upper blade guard is a safety feature. Without it, the blade is exposed. An exposed blade can cause cuts or injuries to the user or bystanders. The trigger lock and other safety features (if present) are separate from the upper blade guard. So, the main risk is the exposed blade.

Answer:

The exposed blade poses a danger.