Interim titles establish who is to be definitively given the next title shot. They say that this guy is the number two in the division. It basically formalises the idea of the number one contender fight. If someone has the interim title, you can't bring in a Nate Diaz or a Dan Henderson for a title shot because you think it'll make more money. Rather than being a negative result of a focus on money over legitimacy, interim belts help to combat this tendency. I think the idea of interim belts, or number one contender belts, should be rolled out across all weight divisions.