Last year in Japan, a special investigation committee to actual status of the assessment system was organized and recently it reported its investigation results.
According to the report, the collusion among certification bodies (assessors), consultants and audited companies have generated a down spiral of credibility of ISO9001 certification. And one of the main reasons is that a company is used to have the registration as a metal, not as a tool for quality improvement.

But I think from my experience another factor of the collusion is lost in the report.
It is behaviors by quality people of large companies who enforce wasteful documents on its suppliers in the name of customer requirements. Even if suppliers have slim ISO9001 system registrated by the third party assessments, the people often ignore the system. The behavior also generates a down spiral of credibility of ISO9001 certification.