You should change the heading to "decathlon/heptathlon." Regarding the pentathlon, I can give you my own experience. The military likes to have a good pentathlon team, because the pentathlon envisions the activities of a great military agent of the past: He could shoot guns, fight with swords, and travel well (to escape, attack, or deliver messages) by horse, water, or foot. Decades ago, when I was in the military, the pentathlon program would seek to recruit athletes who could run and swim distances reasonably well; they figured that they could teach the athletes the more arcane (and less taxing) events of fencing, horse-riding, and pistol-shooting.
Regarding the decathlon and heptathlon, the most important thing to recognize is that one can become world-class using almost purely anaerobic capacities and skills; decent aerobic conditioning or capacity is unnecessary. In the decathlon, the only significant aerobic stress is in the final event, the 1500m. (in the heptathlon, the 800m, which merely adds a fair bit of aerobic stress to the demands of the 400.) By that point, the competition is usually pretty much over, and a poor 1500m runner, like Daley Thompson, could simply jog five minutes or so and still get an easy win. (By contrast, Bruce Jenner, Thompson's great predecessor, was surprisingly good at 1500m by decathlon standards.) The time demands of nine (or six) individual anaerobic track and field events, especially with highly overlapping conditioning and skills development, are not so different from the time demands of endurance events, like the marathon and (perhaps most relevantly) triathlon.