Chins are an integral part of boxing. It is a sport in which the main focus is getting hit in the face. In fact, I wouldn't be surprised if Terry Norris would come across as having a granite jaw had he chose to compete in mma. Boxing purposely restricts many techniques to maximize the possibilities of two combatants duking it out. Dexterity, hand speed, and chins are essential to making it to the top.
MMA is different than boxing. Physical strength can become more of a factor than in boxing. There are many more techniques to defend against. In mma, fighters can clinch after closing the distance and have more ways to avoid getting caught on the chin. That's why some of the fighters you have ranked with the poorest chins around have found away to be on top of their divisions at one time or another. If GSP ever gets exposed for having a poor chin, he'll prove you can dominate the sport without one.
Now can Rashad dominate without a chin the way GSP may have been doing? No, I doubt it. GSP's stand up is tighter than Rashad's, and GSP is stronger at 170 than Rashad is at 205. Rashad would need a better chin if he can't find ways to excel in other areas.
Jon Jones may be someone that can avoid ever having to prove how good his chin is.