I once queried a no further action response to a close pass, in their follow up they mentioned that when viewing my rear view footage the driver's number plate could be seen.
I asked them what they meant by this. They said if the number plate is visible then they are not too close....
In my experience - responses, the reasoning behind decisions, and 'success' in terms of a NIP vary wildly depending on which bit of London you were in when the incident occurred.
As @Oliver Schick says, it's worthwhile reporting so that evidence is available further down the line if necessary and as @dancing james says better technology can mitigate some of the issues as far as subjectivity is concerned.