Alas, despite earlier reports that Mel Gibson would have a cameo appearance in 'The Hangover 2,' he's now been dropped from the cast, apparently owing to objections by Zach Galifianakis and other cast members. Too bad; the jokes would have written themselves.
But then, that's the point, isn't it? We'd have gotten see Mel Gibson laughing at his own misconduct, he'd have proved he's a good sport, he'd work his way back into our good graces. Meanwhile, the publicity/shock value would have gotten a few more people to buy tickets. The whole process worked so well with Mike Tyson's cameo in the first 'Hangover,' and there's no reason to think it wouldn't have worked again with Gibson.
There are a lot of reasons filmmakers seek stars for cameos, and a lot reasons stars take on cameos, and those reasons usually have little to do with serving the story. Some of the reasons are outlined below -- along with some of our favorite cameos.