Engine dyno vs chassis dyno

I used to work at a shop with a chassis dyno. First question after dynoing just about everybody's car was "So, what's that at the flywheel?" Wouldn't you rather know the actual measured power at the wheels than a guesstimate flywheel hp?
Our dyno was an eddy-current machine so we could do part throttle tuning, combine that with the wide band a/f meter, and we could find power for just about any car. And not just peak power, all through the powerband usually.
Our biggest problem was that the Mustang Dyno was very accurate. (read: lower than expected numbers) Nobody wanted a realistic measurement, they wanted "magazine horsepower".