I have never belonged to a church that uses the Sinner's Prayer and I'm not at all interested in altar calls or most of what goes along with that variety of Christianity, BUT I do think that the Sinner's Prayer is too readily ridiculed.
It's not as though this prayer is floating around, printed on a handout or something like that, in case anyone wants at a moment's notice to be saved for all eternity. Just take it out of your wallet and recite it, and you're home free. Or that any time you hear a preacher saying to get saved, reciting this prayer is all there is to it.
The Sinner's Prayer is a simple affirmation of a commitment made by a convert after having been convinced by the Bible, reason, etc. at the hands of a preacher. IF there is such a conversion experience (as was the case with several figures from the New Testament who heard, believed, and were baptized), then the person is encouraged to make a public witness of that commitment. He could use a different prayer, in theory. And then he is customarily directed to other preachers or evangelists--or to a local church--for additional instruction in the faith.
To my mind, it is unfair to churches using the Sinner's Prayer to talk as though there's some 'slam, bam, thank you Jesus' system at work.