The idea that diagnostics are useless contradicts everything I’ve ever learned and taught about diagnostics. 

Diagnostics help identify problems. Diagnostics can turn a fault that is unrevealed during normal operation into a detected fault, which allows me to take action. That action could be an automatic transition to the safe state or the initiation of a maintenance activity. 

The Safe Failure Fraction formula even shows very clearly that detected failures are good: 

So, what could a statement like this be based on? Digging in a little revealed that the user making this statement had issues with several smart components and the alleged diagnostics the components had. This included several smart transmitters, as well as a digital valve positioner. These products were certified by exida, hence the existence and the correct functioning of the diagnostics was proven. 

The problem the end user had with the diagnostics, which led to the subsequent statement that diagnostics are useless, was that the diagnostics were implemented incorrectly. The notifications that were expected did not occur—in fact, unexpected notifications occurred. Though the configuration of the devices was included in the user documentation, this particular end user had trouble understanding the required installation aspects of the smart devices. 

As a result, the diagnostics were useless. But the problem was the inability to correctly install the smart equipment. This could be a combination of complexity of the product and lack of competency to deal with the smart devices at the end-user site. However, this does not mean that diagnostics in general are useless.