Ralph - and all my thoughts on OS and FS would be much the same as yours - for me the primacy of the particular purpose and outcomes desired with the particular client in the particular programme at the particular time, is the key question. Only after real clarity on that can I personally begin to design a programme or any individual event, with the client. That's the point that I would consider whether OS or FS were relevant and the best choice.
It is also as you say, often to do with the personality of both the consultant and the client. Some are more pre-disposed to taking things on faith + a little greater risk. aplogies to all for being so silent for so long - fortunately it's largely to do with have lots of folks wanting lots of Open Space etc. Romy PS see you all soon Ralph