It's difficult to find either primary research or reviews in this area whose authors haven't received grants from companies that make such sport drinks or more medically oriented rehydration solutions.
Taking the 2007 Position Stand of the American College of Sports Medicine (ACSM) to "as good as it gets" in terms of existing reviews in this area, their overall conclusion is that
During exercise, consuming beverages containing electrolytes and carbohydrates can provide benefits over water along under certain circumstances. After exercise, the goal is to replace fluid and electrolyte deficits. The speed with which rehydration is needed and the magnitude of fluid/electrolyte deficits will determine if an aggressive replacement program is merited.
And to pick some detail:
The composition of the consumed fluids can be important. The Institute of Medicine provided general guidance for composition of "sports beverages" for persons performing prolonged physical activity in hot weather (73). They recommend that these types of fluid replacement beverages might contain ~20-30 meq·L−1 sodium (chloride as the anion), ~2-5 meq·L−1 potassium and ~5-10% carbohydrate (73). The need for these different components (carbohydrate and electrolytes) will depend on the specific exercise task (e.g., intensity and duration) and weather conditions. The sodium and potassium are to help replace sweat electrolyte losses, while sodium also helps to stimulate thirst, and carbohydrate provides energy. These components also can be consumed by nonfluid sources such as gels, energy bars, and other foods.
You can already guess from this that the extra stuff in sports drinks, except for sodium (which stimulates thirst) might not be directly useful in increasing hydration but may help with other issues affecting endurance and performance, e.g. carbohydrates provide energy replacement. The Position Stand actually has a lot of material on that. So your question (focusing on hydration alone) may be too narrow in this sense.
Another issues discussed at length in the Stand (with empirical evidence mostly from marathons) is hyponatremia: drinking water (without added salt) may dilute blood sodium concentration too much in some circumstances. So again it may be too narrow to ask if you get enough water replaced.
And a 1994 paper partly derived from US Army experiences, outlined some cases when they thought more than water is needed:
carbohydrate-electrolyte replacement fluids may be necessary in some, but not all, military field situations. The greatest need for carbohydrate-electrolyte replacement fluids is experienced by soldiers who (1) lose more than 8 liters of sweat per day; (2) are not heat acclimatized (e.g., during the initial 8 days of field living); (3) are performing a prolonged, continuous exercise bout (>60 min); (4) skip meals, have meals interrupted, or encounter anorexia because of a hot environment; (5) experience a caloric deficit of >1,000 kcal per day; or (6) are ill with diarrheal disease. The fluid-electrolyte needs of soldiers are specific to the intensity, frequency, and duration of the exercise involved, as well as the environmental stress encountered. This information is not presented to imply that many different solutions are required, but rather that the best use of such fluids can be recognized with proper soldier training (i.e., when to use them) and simple instructions for field implementation. It appears that carbohydrate-electrolyte replacement fluids, like weapons, should be available to the soldier for use when needed.
The US Army might not have found such drinks to be cost-effective though. Their 2003 revised hydration guidelines (Kolka et al.) managed to reduce the experimental incidence of hyponatremia simply by reducing the water intake (and did that without an increase in dehydration).
Finally Tim Noakes has a fairly critical review (2010) of the ACSM standards (including the 2007 one) and of the older (1980s) US Army standards, particularly as it concerns the amount of fluid intake recommended, which is excessive in his view. Noakes' review doesn't cover much the add-ons provided by the sports drinks, but he is quite critical of influence that the sports drinks industry has had on research. He says for instance that hyponatremia is largely a manufactured problem by standards suggesting that athletes drink "ahead of the thirst", that is more than they feel necessary (ad libitum). He does concede that the sports drinks carbohydrates' add-on do provide some performance benefits, e.g.
[Dr. Cade's Gatorade] simple studies at the time  did not show that fluid ingestion was necessary to prevent heat stroke during exercise. However, the ingestion of the carbohydrate-containing solution clearly improved performance in a 7-mile run/walk, at that time a novel finding.