One of the big problems with Flattrac data is that it is measured at slow speed on a belt sander. Then when you get into the real world you get to apply 'corrections' to that carefully measured data to make it match what you measure on the road.
Now, if these corrections were mostly in the non linear range, and say 10%, you might think there is some integrity in the process. Sadly we often have to correct the linear range, by as much as 30%, to get good correlation. Now when I say good correlation I mean anything from overlays of parking effort forces in the tie rods, like this (on concrete and a slip plate) used for designing the PAS
http://www.mediafire.com/view/itsv3imir9g0xu3/PE.JPG
which admittedly doesn't need an ADAMS model, so long as you can predict the slip plate curve (the small one) in your spreadsheet
-to being able to predict the steering wheel angle at which a SUV will start to rollover, at a given speed, and correctly model the post-wheel liftoff behaviour - ie does it roll or does the wheel touch down again? Or in crosswinds, if the car is hit by a blast of wind when passing a truck is it going to be stable?
All of these things can be estimated in simpler models than ADAMS, but perhaps not as accurately, and almost certainly with more effort, once you have the ADAMS model built. And sadly the tire is probably 50% or more of the correlation work.
But for a race car, where frankly if the coil spring needs to be made 20mm bigger in diameter you can just do it, is there as much value in this?
...well, yes there is. On an open wheeler it is crucial that you have linear range understeer in a straight line, or at least, your instability speed is greater than the max speed of the car. Your tire data is the only way you can work this out in advance, especially if you have aero.
You don't need a fancy ADAMS model (the equations are in Bundorf's SAE paper, or probably RCVD), but you do need to know how the cornering stiffness varies with vertical load for each tire. I have also run into problems on a production car where a tire did not have the same sensitivity as the one it replaced - we were using that to get limit understeer, the new tire didn't saturate, so the car would oversteer at the limit, which is not acceptable for a road car these days.