Well, on the side of the defender, (and this may sound silly) but I don't like to assume that my attacker is wearing a shirt. For that matter, I don't like to assume that my attacker has hair. They very well may not and I'd like the practice to not be limited to that. From the side of the attacker, I find that it is easier for most people to defend a grab to clothing rather than a grab to limbs and flesh. For that reason I like to teach to defend against a situation that I find to be a more difficult one. Keep in mind that I do not regularly teach Hapkido, nor are my students regular students of Hapkido. If I was teaching a complete Hapkido program I would certainly teach to defend against both situations.