Form together to negotiate on an issue, and disband.

If the Union just sits there as an organisation existing, then it's defunct.

Is there a legitimate way to represent the working man's interests

Negotiate with your Boss. If you're in a job that you're replaceable, know your negotiating power is very limited.

and/or protect him from his employer's bad behaviour

Laws.

If the laws don't exist then form a union to lobby for a law preventing spinning horizontal saw blades at neck height on the shop floor.