Based on what I've seen, refusing to add someone can be seen as a social snub in many circles. Simply adding and muting someone avoids a lot of social hassle of people wanting to know why you refused to add them when you did add some other person.
This is one of the things Google+ got relatively right: Perceived "friendship" are not in any way symmetric. Even close friends will often have different ideas about the depth and importance of their relationship. Facebook has tried very hard to ignore that.
