Reg, do you remember in the NPTC stuff that top handled saws could be used with one hand as long as there was nothing in the kickback zone...rope, arms etc.
I think the standard was to use them with two hands at all times...unless by doing so it was then unsafe.
On a standard setting event I went to the guys did an actual climb and cut and demonstrated the situations where one handing for the purposes of the assessment was ok. It seemed a pretty sensible approach to me.
We all know after the fact of training (and assessment for some) we do things that are not strictly 'by the book', but we do so from a position of knowledge, we know what to anticipate for that could go wrong so hopefully when we step outside the box we do it with our eyes wide open!
Personally I find it a bit hard to one hand a 200 and I would not be able to control it if it kicked, so I do it when the cutting is level or below me so gravity is my helper. The 150 is a different think, I have to be sensible about one handing it because the ease with which I can do it has the potential to lead to complacency and maybe a major OOPSIE.