The second usecase - giving points to students who entered the correct value with the wrong decimal separator in calculated, simple calculated and numerical questions - is no longer an issue. This is fixed at least since Moodle 3.0.4.
However, there is another usecase where this feature would be helpful. There are question types (e.g. qtype_kprime) where students have to select options as in multiple choice questions. Even if teachers try hard to create unequivocal and unambiguous options, they sometimes only realize after the exam that an option specified as wrong in certain cases might be correct. To be able to give points to those students who selected this option, manual grading is needed.