According to ASME B30.10, a latch is a mechanical device used to close the throat opening of a hook.

The purpose of the latch is simply to reduce the possibility of the rigging becoming detached from the hook, primarily in slack conditions. The use of a hook with a latch does not preclude the inadvertent detachment of a slack sling or a load from the hook.

Hook latches aid in the retention of loose slings under slack rigging conditions only and are not intended to be anti-fouling devices during load handling or rigging. A latch is not intended to, nor rated to, carry a significant load. [Source: ASME B30.10]

Crosby’s recommendation is to use a latch, with very few exceptions. The standards tend to compel the use of a latch. OSHA expects to see a latch if there is a place to attach one. The use of latches is clearly a best practice. 

Crosby advises to always consult and follow the guidance offered by ASME, OSHA, and other safety administrations.