Many thanks folks, esp BIK.
I have a working knowledge of this now. Though I think it'll be a short while before I have an understanding.
I agree with Imabell that in principal, any pax at all should be afforded the same level of care, how many of the pax are able to quantify the risk of a particular flight?
...and if private pilots aren't bound by flight and duty times,
then safety is likely to be compromised.