That finding comes from a study at the University of Iowa (via BoingBoing), entitled, "The Demand For Aggressive Behavior In American Stock Car Racing," published in the journal Sports Economics, Management, and Policy (subscription required), which took a look at 135 NASCAR races spanning the 2001-2009 seasons. By evaluating the television ratings through a range of factors, the study sought to determine the effect of crashes on viewership.
While crashes did increase viewership (by about 6 percent per crash per race), the Chase for the Cup scheme introduced in 2004 threw a wrench into the old formula. From 2004-2009, the Chase itself was found to bring in viewers and increase excitement, by eliminating the chance of one or two front-running drivers pulling so far ahead in the season points as to make the outcome of the season-ending races relatively unimportant.
The effect of the Chase was to essentially extend the competitive season, or, as the study authors put it, to create a "second season," where the top dozen drivers could battle it out--drawing in more fans of more teams and drivers. Now, it's the size of the points gap between places that drives viewers, not the crashes--though they still help.
According to the Iowa study, a 50-point decrease in the points gap with 20 races left in the season raises viewership by 4 percent; the same reduction with just 13 races left boost viewers by 6.1 percent.
Obviously there are two camps in NASCAR fandom, but it looks like those wanting a clean, close fight to the finish are winning against those that just want carnage.