Very often user want to subcribe only to the particular discussions instead of the whole forum. Its particulary true for the problem-related forum in courses, or to the problem forums on the main moodle site, full reports from where can flood e-mail box.
Teacher also may want to receive messages only from his students from group, not from all students in the course.

(I just voted for http://tracker.moodle.org/browse/MDL-7368 but realized that it only applies to v. 1.6.3, that's why I made this new entry)