A secondary duty is an opportunity to display an attribute that may not be apparent in a primary role. As an RO you may not necessarily observe this first hand but should comment on it from evidence obtained from other sources. (OIc, success of the Summer Ball, football team etc). Similarly an OOA report is an observation of your subjects performance in his/her primary (and possibly secondary) duties in a different environment and should be used as evidence in your assessment. Ditto post training reports (F292 for OR ground trades)
If you don't do it this way then a sizeable chunk of the an individuals reporting period is lost.
And has it occurred to you that most people get a good OOA report because they rise to the challenge and actually deserve one? I've found that people do the "work hard play hard" bit quite well when given the opportunity.