Mirror for a Blind Spot.

A complainant allows you to outsource part of your thinking for free.

A complainant gives you access to an argument and reasons and even facts and evidence you may not otherwise consider.

A complainant is a camera on the dark side of the moon.

A mirror to your blind spot.

A complainant provides free consulting services to add value to your decision making.

If a person with skin in the game cannot give you better information than you already have - you can be even more confident in your decisions and actions.