to pretend to be against an idea or plan which a lot of people support in order to make people discuss it in more detail and think about it more carefully
Usage notes: The 'Advocatus Diaboli' was a person employed by the Roman Catholic church to argue against someone being made a saint (= someone given the honour of being called Saint by the Roman Catholic church).
I don't think he was really in favour of getting rid of the scheme, he was just playing devil's advocate. I know that most people here support the project, but let me play devil's advocate for a moment and ask if anyone has considered the cost?