Not sure of how would be the performance impact but sometimes we just need to know if the callback exist, not execute it (we would require to include the file with the callback for that) so would be better to read the cache instead of a big lib.php file or any other big lib file. I thought about it last week while reviewing something, now I see it again used in grade_item::adjust_raw_grade, although doing a git grep component_callback_exists I don't see as much uses of it as I thought.