After a roof has been pressure washed, it looks brand new. However, the roof is no longer as thick as it once was. Pressure washing tears away the outer layer of wood cells, forever making your roof thinner by anywhere from 1/16" to 1/8" removing valuable years of life.
When your roof is pressure washed, the water is sprayed on at about 2000 psi or more. It the past the shakes used were thicker so this stripping away of wood was not as much of a concern, but today most shakes used on roofs are called mediums (thin shakes). The shakes used today that are classified as heavies are still not as thick as the heavies of 15 years ago.
Given the thickness of the shakes and the amount of water pressure being sprayed on, pressure washing cannot only cause leaks but can also result in doubling the repairs needed on your roof.