Besides the other bone weights, do all armour/clothing parts have to have BP_TORSO, BP_RIGHTARM and BP_LEFTLEG?

Can I just have BP_TORSO for body armour, BP_RIGHTARM for gloves and BP_LEFTLEG for boots?

What happens when one of them is omitted from a piece of armour/clothing?

Well, first of all, Skyrim meshes do not have BP_Torso, BP_Rightarm, BP_Leftleg dismemberment parts. Those are old Fallout names. They are now properly named as SBP_32_Body, SBP_34_Forearms, SBP_38_Calves, as well as many other parts. I have a http://tesalliance.org/forums/index.php?/tutorials/article/111-creating-skyrim-armor-in-blender-part-1-understanding-skin-partitions-nifskope/ that can explain it in greater detail.

You could just use SBP_Body for armor, but you run the risk of creating gaps/missing body parts when people start to mix your armor with other armor pieces.

Nice, thanks for the answer and the tutorial!