I said this way a basic test to figure out your fiber composition, generally speaking. While a skeletal muscular biopsy is much more accurate, many people on these boards do not have access to those types of tests performed.
It also depends on the type of lifting the individual performs. For general strength, muscular hypertrophy, and overall fitness, a variety of mixed repetitions would be beneficial. For a bodybuilder or powerlifter having a variety of mixed repetitions would not be as beneficial, especially if that individual is planning to compete. Also, it is not possible for someone with a high composition of type I fibers (even with pure strength/power training) to lift as much weight as someone with a high composition of type II fibers. Nor is it possible for an individual that is highly composed of type II fibers to perform high endurance events. While training can alter the biochemical properties of muscle fibers (typically type IIa) they cannot perform exactly like a type I or type IIx fiber.
All fiber types (type I, type IIa, type IIx) are metabolically enzymatic. Type I fibers being more aerobically oxidative, and type II fibers with LDH, ATPase, and Creatine Kinase activity--you refer to them as 'myosin heavy'. Type II fibers have 8 different contractile proteins, myosin isn't the only one; however, actin and myosin contribute the most to muscular contraction.