@drop
the Rating scheme must have more than a few gaps.
Yes. As mentioned, nepotism & the buddy factor are always in play with JW privilege distribution.
Another factor is, as much as many elders would relish hoarding most of the speaking parts for themselves, that is unsustainable, so they become forced, perhaps even reluctantly, to spread the wealth of weekly speaking assignments to other elders/ms, mediocre public speakers as they may be, because delivering public discourses becomes burdensome, especially when frequently traveling out to other congregations. It is time consuming (and increasingly fuel costly) to prepare, travel out, deliver discourse, then travel home, sometimes with additional responsibilities waiting for attention at the home congregation meeting, such as an elder's meeting after WT study.
Including mediocre speakers on the scheduling list helps to lighten the load for other elders that have learned the wisdom in selective delegating.