1. The body adapts to the specific stimuli it is exposed to. If weights are heavy, muscles and bones become stronger. Strength and bone density are far more important for health than muscular endurance. Leg strength is also an independent predictor of lifespan (Newman 2006 J Gerontol A Biol Sci Med Sci).
2. Lighter weights will not train the fast-twitch motor units that people lose with age. With the loss in these strong, powerful motor units, people gradually their ability to live independently. (Dr. Phillips disagrees with this - he believes that with exercise-induced failure, one recruits higher-threshold motor units).
2. Total work = load lifted * number of repetitions. A higher load will increase total work just as more repetitions will. A higher load also increases the post-exercise increase in energy metabolism (the primary manner in which RT burns fat) more than low-intensity will (Thornton MSSE 2002).
3. Fatigue from multiple, low-intensity repetitions results in decrements to proper form (Hooper 2013 JSCR), which is what results in injury. Heavy weight is not the primary concern for injury.
4. The most important point here, however, is that people should train in a variety of different intensities to develop different muscular abilities: strength (~3-6 rep zone), hypertrophy (~7-12), and power (light weights for few reps). In my opinion, this approach is also far more engaging over time.