I remember reading somewhere that top Swimers, Soccer players, and Boxers would get down to about 7 to 5%. I would imagine since they train constantly for months at a time to get into that shape, its is not so much that it is unhelathly but just takes a lot of effort to keep your body at that level. In a way, your body is fighting against you to stay at those levels just because it puts so much of a strain on on it. Your body wants to take road of least resistance. I guess by gradually getting to these lower %'s, you can trick it into accepting them as normal. It seems that too many people try to get down to those levels at a speed which is unhealthy.
But that is just a guess.