How do Pacesetters Affect the Outcome of Races?

As a person who watches sports events and tries to put in some analysis about what it takes to win one event or another, I am still unable to understand fully how record breaking performances benefit from pace setters or not. For instance, Sammy Wanjiru is without doubt a phenomenon in the marathon. I first watched him run during last year's Olympics games in Beijing and was impressed with his control of the race and the calculation towards the ends of that race. My thinking then as he casually jogged into the stadium was that this was definitely a unique runner among elite athletes in an extremely difficult and painful event.

Apart from that performance, the same athlete handily won the Chicago Marathon a couple of days ago. Liz Robbins of the NYT reports on the race and mentions the progress in that race. The fascinating thing in my view is the versatility of the athlete here because the running conditions in Beijing last year where the temperatures were very high. Come Chicago a year later and in vastly different conditions, he still won impressively.

The lingering question in my mind though is that the races are also different because unlike the elite marathon events, the Olympics event does not allow for pace setters. I wonder what effect, if any, the pace setters really have on the outcome of a race and whether there may be a better device for pacing an athlete towards breaking the record without involving other runners. In my view, the use of pace setters involves the introduction of assistance during the race. Not that it would matter for Sammy Wanjiru who lowered the record in Beijing by three minutes despite the absence of a designated pace setter. And noting that pace setting is not possible for the sprints, are there conditions in which pace setters are most siutable?