Not necessarily a tool for the slackers. People who have read the actual book can also use it to get grounds for their ideas, see important themes missed, and get aquinted with the public opinion; so that by the time of the actual test, one's opinion is supported, strong, ellaborative and analytical.
Besides, it's written in good English and is proven to be surprisingly reliable; with the exeption of some trivial tries for 'abstract' thought wandering, which could be easily spotted, anyhow.
Why wikipedia when you have SparkNotes?
Why get a B when you have SparkNotes?