I think the number of wrong airfield landings are statistically interesting because they are the only non-deniable indications of pilot error. If this number goes up you can bet that other mistakes have increased. In the same way, the number of diverts is a good indication of serious mechanical issues. I guess the number of runway excursions of a given type also serves as a canari.
In fact these are the only "incidents" where the public can see imeediately that something unplanned happened. Every other type of incident (mechanical failures, pilots falling asleep etc) is only visible to industry insiders, and in the case of non US airlines I am not even sure that reports are always filed.
So I believe that the number of wrong-airfiled landings can be used as a basis for seeing whether it is a "safe" or "unsafe" year. The public will never see the true stats on the number of bird strikes, uncontained engine failures, or on the number of overtired pilots who make a recoverable booboo.
Edmund