Please note the fundamental differences between the two systems:
1) Scoring guides are always based on standards that define performance in functional terms; letter grades are often based on point scores, with points that may not refer directly to functional competence.
2) Tests and other activities that yield letter grades are often used to generate a "bell curve" that is in turn adjusted up or down to yield a desired distribution of grades. Scoring guides measure how many examinees meet or do not meet a standard, with the score of 4 defining the border. In my courses "sufficient" is a B-; C+ students are considered ready to go on for more learning too, but they are not always comfortable doing so.
3) Scoring guides imply that student learning for a given standard is assessed more than once, if necessary, with revision and rescoring as basic parts of the system. That is why my courses invite you to revise your work, sometimes even your tests, for rescoring and appropriate change of grade.
4) Scoring guides imply that learners take part in their own evaluation and also take more responsibility for their learning. Letter grades may create an atmosphere of rote learning, mysterious tests and grading, and passive learning. That is why may scoring guides for my courses are publicly posted; you are encouraged to use them to guide your learning, even before your performance is assessed.
5) Scoring guides imply that learner placement is important. You are encouraged to ask for placement advice. Scoring guides also imply that learners who can move ahead should do so. Unfortunately, we don't yet have a system in place that allows for fractional moves, such as from 101 to 102 within the same quarter. We're working on it!

Scoring guides are being used more and more as education is modernized to serve a society that needs large numbers of highly-trained citizens. In such a society, learners who do not meet standards are given more attention; those who readily meet standards are encouraged to move on to new learning. The A-F "bell curve" system implies a society where all learners receive the same amount of instruction ("seat time") under the same circumstances. Then a test is given, and elite (or perhaps just privileged) learners are identified and selected for further education and, eventually, privilege and power. The others (the larger group) are confined to "lower" levels of the area, redirected to other subjects, or culled from the educational system. In the agricultural or even industrial past there were more such jobs, and some even paid fairly well.

An economy based on services and information needs a better educational system than one of seat time and the bell curve. And instruction and learning that are defined by standards tend to provide more rewarding educational experiences.