I've heard a lot of reasons why Rampage should or should not take the fight with Evans at UFC 98, but everyone has missed something. If Rampage turns down the fight, he's looking at an extended vacation. The absolute earliest he would get a title shot would probably be in September. This would mean six months off at the very least.
This has nothing to do with cage rust and everything to do with money. Fighters do not get paid a salary. They are paid to fight. Does Rampage want to wait half a year or longer for another lump sum of cash? He also might find himself in a less lucrative position, if Machida wins. Rampage/Evans is a huge bout, which will draw a large PPV audience. UFC 98 also sports a solid card. It's tough to walk away from that sort of thing, especially with the uncertain economy. Maybe six months from now the UFC's deal with the devil expires and the PPV numbers level off. Rampage's decision to wait could end up costing him a lot of money. And in the end, it always comes down to the cash.