There has been some conjecture that knee braces, while saving the knee joint in accidents that would otherwise have injured the knee, may contribute to fracture of the leg bones instead. The theory is that in a major leg-bender that without the guard, the weak point is the knee joint, which gets torn and injured when the lower leg is forced beyond its safe range of motion. However, with the brace on, the knee's safe range of motion is preserved, but the leg is still moving. As a result, the leg bones, either above or below the brace, can break. You would see breaks right at the uppermost or lowermost brace strap.
I think Dr. Mark has an ongoing study of this possible phenomenon. It seems to make sense, but there are no hard facts at this point. However, I can speak as a first-hand witness to a broken leg by a participant at a recent S. Jersey enduro. He had braces on, and his leg break was right at the lower straps of his brace. Would he have simply injured his knee rather than getting a tib-fib fracture if he had no braces? Food for thought. I'm anxious to see Dr. Mark's conclusions after his study.
All that being said, I think someone with normal knees might not want to wear braces. If you have a history of knee injury and minor tweaks can result in further damage, it might make sense to wear braces.