Hey Rob,
Totally agree the real flight loads are not the same. With a fastened heli, you are actually putting more strain on the entire system than you would in a hover or forward flight. This is especially true when running at full pitch/throttle and while holding in full tail rotor pitch too giving maximum engine loading. This makes high speed idle setting actually quite accurate and able to handle the most aggressive flying without running lean.
My biggest tuning pain has always been mid range and running too lean while the rotor transitions or during fast descents when I am pulling negative pitch. A fastened heli certainly doesn't reproduce this 100% accurately, but it gets it pretty darn close and in the ball park. Fine tuning will always be required. As you know, even fuel and weather conditions can have an effect, everything about tuning a helicopter engine is a compromise.
All that said, it is a dangerous method, and you are correct to point out it has limitations.
Cheers,
H.H.
www.rchelicopterfun.com