Skytorque,
Yes I am aware that some light (and usually older) helicopters use a centrifugal clutch for starting, then hold constant engine RPM with changing power demands. But this constant RPM regime is less flexible than the free turbine system. This is why most heavy (and modern) types use free turbines.
Torquelink,
In some cases free power turbines can be slower to respond. But consider how a fixed shaft responds if the rate of collective increase is really excessive. A stalled/surged gas generator responds very slowly indeed (if at all). Even free turbines engines can stall/surge of course, but at least they are not tied directly to the rotors.