Brian,
All that you have demonstrated in your most recent post is the need to be very careful in composing posts to pprune threads.
Your statement that "Free turbines do indeed need clutches, to cater for the engine failure case, if for nothing else." is not true.
I cannot think of a single free power trubine turboprop engine which has a freewheel unit or clutch. It is of course possible that some do, but most do not.
The question of whether all helicopters require clutches depends upon the definintion of a clutch. And as we have seen, different people use different definitions. If you re-read all of the posts in this thread I think that you will also find that most contributors do not use your definition of clutch.
At the end of the day the original question was "what is the main advantage of a free power turbine". I believe that I have already answered this. The greatest single advantage does not require any reference to clutches or freewheel units of any kind.