While true, "the PM may not prioritize fixes", a big aspect you are missing is, someone internally with a problem has access to a direct communication line with said PM, his peers, superiors, etc.
For most day to day usage issues, dogfooding is a great way to hammer out issues. Will they find issues with every use case? No, but they will find some. Also, generally, the more diverse and large the organization, the more use cases will be dogfooded (assuming the bulk of the org is encouraged/required to do so).
When you are talking a company of 90k+, encouraged dogfooding will hit a lot of different use cases.
For most day to day usage issues, dogfooding is a great way to hammer out issues. Will they find issues with every use case? No, but they will find some. Also, generally, the more diverse and large the organization, the more use cases will be dogfooded (assuming the bulk of the org is encouraged/required to do so).
When you are talking a company of 90k+, encouraged dogfooding will hit a lot of different use cases.