Useful for mapping and checking fuelling only IMO
Power runs can be waaay off and comparing different runs on different days on different rolling roads is pointless
A minority I would think,
Use them for what they are, as a valuable tuning tool that is certainly repeatable to within 5hp every-time when the operator knows what he's doing.
Munday, I agree with your initial sentiment; though as far as different readings on different days....a dynamometer has a correction factor that is part of the tuning tool.
So that everything is corrected (certainly in most modern dynos) to either J607 or the European equivalent (ECE/DIN/ISA) which is virtually the same, or the newer J1349 standard which would read about 4% lower than J607 and it's equivalents.
This way when an engine, for example is prepared by a workshop in a sub-zero climate it can be compared to an identically built engine, built to the same specs by another shop in a desert climate and they would show identical power curves.
Then they'd be able to send the engines they built to each other and then dyno'd again to the same standard correction & show identical power again.
It can be done if good standards of control are adhered to.
Though it all starts to get confusing when different
types of dyno are used. As the USA mainly use inertia dynos (Dyno-Jet and Superflow) and EU and a lot of other countries use the eddie-current type and there can be between 12 and 20% difference even with proper control of the correction factor being applied accurately between these types.
If a person were to make use of a dyno, it is best to stay with the same dyno and input programme each time, as then the value of it when making modifications becomes particularly helpful because it will show the differences you've made to the car faithfully this way.
Comparing results from different dyno's is where most of the confusion comes from as correction factor with quite a few shops that are aware "horsepower sells" think up their own versions of correction factors that are more closely related to marketing than reality.