zDom, why do you suppose that they don't need to worry about it in mma? Is it due to conditioning, or the skill level of the people it's being thrown at being able to defend against it?
How good of a kick does it take to cave in a rib cage? So far, GSP hasn't been able to do it with a spinning side kick, Cung Le hasn't been able to do it with his side kicks (which were arguably the best side kicks in San Shou).
I think that in SD, it can be extremly useful and even break ribs (seen it done before), especially against an average person. In MMA competition, where you're likely facing a well conditioned athlete with sound skills, it doesn't work nearly as well and leaves you open to having your back taken or being taken down.
Again, YMMV