Sorry for the convoluted language of the question (I'm slightly delirious right now), but I'm wondering exactly why the current grading system arose, and why it's so prevalent today. It would seem to me that this largely inadequate form of assessment would have been discontinued by now, but it's still the dominant practice in Western culture. Thank you.
Actually, I'm not sure that there is a dominant prevalent grading system in Western Culture. Certainly in North America, the letter/+/- system seems dominant at secondary level and the GPA system is almost universal at tertiary level, but that's really confined to the US and Canada.
In the British Isles, for example, there is a simple letter grade system at secondary level (i.e. just A, B, C, no +/-) and an archaic class system at most universities: most students are graded on a scale between 20 and 80 (pass mark is 40), which are then divided into degree classes (I, II.1, II.2, III). In France, students are marked out of 20, with a passing grade of 10; in the Netherlands, the marks are out of 10. In Germany and Austria, if I remember correctly, the scale is 1-6, with 1 being the highest grade and 6 the lowest. Countries in Southern and Eastern Europe also have different grading practices, although I am not aware of the specifics.
While there have been calls for the GPA system to be extended worldwide to help compare universities across the globe, there is little sign of this happening, with, in the UK for instance, only a handful of (minor, unprestigious) universities making the change. So while I'm not sure where these grading systems arose from, I think the assumption the question is based on is false: instead, we must ask where, when and how each of these individual national grading systems were developed and why they were chosen in each particular case.
It's a questions I'd love to know the answer(s) to, and hopefully someone will be along shortly to do so.