They pit two people against each other because it is supposed to be a debate, but what it also allows them to do is limit the talk time of the other less desirable candidates. Donald Trump was a sideshow but was afforded more serious discussion than Ron Paul. He was even referred to as a possible frontrunner. In hindsight, it seems silly but the media attempted to run with it. That's how they peddle influence. They float little trial balloons, see if they can get any bites, and if they do they expand on it. If not, they move on and prop someone else up. To them, it doesn't really matter which one of their pre approved candidates gets elected, as long as they play a role in getting them elected. This latest nomination period is a testament to that:
First it was Romney, then Trump for about a minute, then it shifted to Bachmann after Iowa, then Perry, back to Romney, and now Cain. Each and every time one of them is propped up it is done in such a way to gauge sentiment. No matter who they prop up, they are still propping someone up when they should remain neutral. It is completely dishonest and, imho, a classic psyop. It is not, as you contend, done strictly for entertainment value (although they have American Idolized process, that much we can agree on).