Slots are used to group attachments where only one item in the group is visible at once. This allows you to have a draw order list of slots that is much shorter than if you had a draw order list of ALL attachments. Creating a slot per attachment defeats the purpose of slots.
It does make sense to add slots when a skin requires it. Ideally it's still done in a generic way that could be useful to other skins, like calf-over-feet and calf-under-feet rather than elf-calf. Some slots may be better as belt-item so they can be used by multiple skins rather than elf-belt-dagger. When it's absolutely required to have a slot that no other skin will ever use AND another slot can't be repurposed, only then would you name it something specific to a skin.
A slot has no notion of position. They important aspects of a slot is which bone it is under and where it is in the draw order. Therefore if a slot is on the right bone and in the draw order where you need it, then you can use it for any attachment. It may make sense in a highly customizable skeleton that contains skins for many characters to have slots like belt1, belt2, etc. You may want to name some slots based on draw order like belt-under-shirt. A skin could use these for any attachments on the belt bone, without needing a slot for each skin attachment.
The entire draw order must be valid at any given time. If we hide some slots in the draw order, those slots still exist and still need to have their position in the draw order defined. If you change the draw order, the slots you can't see likely get left in some draw order position that doesn't make sense. This doesn't seem very good. It is better to see and think about the slots that other skins use, unless you are making animations that are really only for one skin.
Using folders (in 4.2+) you can group your slots based on how you plan to make draw order changes. This might be by limb/hand/foot/weapon/etc. If you have many slots because you have many skins that need them, this reduces the number of other slots you need to think about at once. Folders coupled with using fewer slots as described above should make the draw order manageable.
