The fact that you have such a large number of laptops in your possession suggests to me that you're probably in a corporate IT setting (apologies if I'm wrong). In such a case, Hardware failures are probably going to be the most prevalent mode of failure for needing a system re-image, since software usage is going to be in-line with corporate policies. People aren't going to venture into warez sites, etc.

But for the average, non-technical user, I think you'd see a much higher incidence of needing a system re-image due to viruses, malware, spyware, etc.