I think the idea of a spoiler spoils the show. It stinks. Just think...this person comes on well rested, not having to endure auditions, weeks of boot camp, being housed with the other contestants, away from family and friends for weeks, didn't have to quit job, etc.
They will get a free pass to fame . What a stupid idea. Maybe these finalists should have skipped everything and just applied to be the spoiler. Then they could come on and just have to deliver one great performance, not a solo and lots of group numbers.
The more I think about it, the more it stinks.