Amongst people who are not double jointed, only one in fifty will escape some form of rheumatic or arthritic complaint in their lifetime. Double jointed children are in danger from well meaning parents, teachers, and others who by discouraging the child from exercising its joints through their full natural range, cause the development of inadequate cartilage protection, and inevitable joint problems in later life. Some use the term hypermobility as a descriptive diagnosis. But it is a useful diagnosis because it identifies the need for a particular exercise regime, which will be largely unknown by many physiotherapists and most exercise specialists.
Medical terminology is used by convention and is evolving.
There is not universal agreement about hypermobility. Hypermobility is not a syndrome. Hypermobility is not considered normal, but neither is being tall and thin. Being tall and thin is not a symptom except when it is caused by a disorder. Hypermobility is more complicated and it might not be a direct symptom of any syndrome.