Air filter intervals really depend on conditions where you drive. In some areas, with a good amount of rain, air filters are probably changed more often than necessary.
If you do much driving in areas like west Texas it becomes more critical. They use an extremely hard caliche clay to surface dirt roads there. When it breaks down it is like talcum powder and gets into everything. Most of this damaging "grit" cannot be seen very well.
I have heard from several Powerstroke mechanics of "sanded" turbos due to owners not changing air filters because they did not appear dirty.
If you have been driving in dusty conditions, it really makes more sense to me, to change the air filters even more often. Air filters are much cheaper.

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Sam

84 300SD 350K+ miles ( Blue Belle )