Well, my take on it is on a diverse board like 460ford.com is how would one level the field between those that drag race, those that daily drive, the ones that go mudding in trucks, power their boats with it, etc?
Obviously the chassis dyno wouldn't do.
However almost anyone building a motor can relate to what it puts out on a engine dyno. Do you discount the guy's combo that is running a 600hp flywheel in his 4x4 truck and is only putting out 425 rwhp at the wheels compared to the guy in a late model mustang who has the same combo and is laying down 500 rwhp?
Secondly, the scientific method would dictate to limit the uncontrolled variables. On a chassis dyno there are a number of elements that cannot be adequately controlled like driveline efficiency, wheel/tire combos, hell even wheel alignment and tire pressure will throw the results off on a chassis dyno. This forces people to then pontificate on the value of certain parts and the value of one person's dyno sheet over another. I can't tell you how many times I've added a part on my motor and gained 8 hp on a back to back chassis dyno run and haven't been sure whether the gain was just due to standard deviation or even just increased thermodynamic efficiencies.
Now I'm not totally discounting the value of the rwhp chassis dyno. I'm just saying that engine building guys trying to eek out the value of one modificaton over another will not be able to accurately do that without doing it on the engine dyno. That's why Nascar uses the engine dyno to ensure the highest level of accuracy, repeatability and test throughput.
'68 Galaxie XL 500 Fastback - 514 Motor Build in process, TFS Streets and intake ported by RHP, XR292R Cam
'02 Harley Davidson Edition F150 Supercrew
Built 5.4 Motor with 2.6 H Kenne Bell Supercharger