I believe the thinking behind this is that on the road you have 1 contact patch per tyre, obviously with two rollers per wheel, you have two contact patches. And if strapping down is such an issue, then that surely applies to all rolling roads, as a car doesn't need to be strapped down on the road.... (I know aerodynamics come into play on the road )

BUT, as for 'true readings' on rolling roads in general...... that's a whole other subject in itself, regardless of type.

As for Cosworth etc. I'm pretty sure they use engine dyno's for development purposes. If they use RR's, then that would probably be for comparative purposes and in conjunction with engine dyno results, not 'power runs' as we know them. (And which part of Cosworth??)

I guess the source of the quote would be pertinent too....

EDITED TO ADD: I'm no RR expert