I can see the usefulness of a focus lock in several scenarios.
With a handheld wide angle lens for zone focusing, especially in crowd situations where the focus can easily be bumped-off from where I set it.
In a landscape setup where the focus is set and now just waiting for changing light, clouds, etc.
Any locked-off setup where an accidental brush of the hand against the lens barrel could knock the focus off.
Panos, as Akira has mentioned.
Close-up setups (like slide duping) using bellows or extension tubes, where the lens barrel is not used for focusing due to the close range.