Dont agree with this one, jon jones accepted shogun on a short notice and absolutly dominated him, people arent interested in a remath between the 2 of them a year after.... imagine theyd be forced to do a rematch right after their first fight
The main incentive behind that is because this way no champ would be ducking challenges. Having your opponent you've trained for be replaced by a random fighter in the last 10 days is a valid concern for any champion. Plus, whoever steps in really didn't "deserve" the shot to begin with, they just happen to be available and willing to fight. So with that in mind, the champ will accept anyone since he knows he'll still keep the belt. The contender can boost himself to #1 contender from middle of the pack by beating the champ while making title fight money (which would be a big jump for a lot of them).
More important than money though, a lot of fighters would love to have a best out of 3 or in this case a min of 2 fights guaranteed IF the short notice fighter wins vs the champ. The MMA game has evolved to the point now that the more they study each other, the more they can make adjustments. This is why I feel having 2 fighters fight at least twice will give a better judgement for the winner. Instead of a random flash KO win etc.. I still stand by the fact that the short notice replacement really has no business getting a title shot, if they did they'd be the #1 contender fighting legitimately. Just because they're stepping in as a favour shouldn't seal the belt for them. Being paid title money and then get boosted up towards #1 contender should be more than enough (especially if they're stuck in the middle of the pack purgatory).
Lastly, the champ has a gross advantage over the short term reps. Even if they didn't prepare vs random fighter X, their body is ready to go for fight night. Most of these random replacement fighters are out of shape and certainly didn't have any training camp to prep vs the champ. So odds are the champ will usually if not always win so the rematch worry is almost non existent while fixing the whole ducking issue.