Putting Kimbo on TUF, a show that's already known for fighters such as Danny Abaddi, Wayne Weems and Blake Bowman, is a win-win move for the UFC. Both the Kimbo fans and Kimbo haters would make sure they didn't miss an episode.
Putting Kimbo on a main event against Lesnar or anyone else might attract good ppv sales for that one event, but will bring into question the UFC:s role as a serious MMA organization.
For Kimbo himself, being on TUF is probably not an option, since he can actually get paid to be elsewhere, and the only thing that can happen on TUF is that he loses even more of the hype that might still be surrounding him.
Maybe Kimbo would agree to get thrown to the wolves at a UFC main event if he gets paid enough for it, but I think he'll also get offers from someone who wants to continue what Elite XC was doing, or from some "pro wrestling" organization, that will keep paying him for a longer amount of time, so it might not be as easy at it seems for the UFC to get him.