Majicpanda wrote

Ultimately, do you feel all of this is absolutely necessary or what is the draw back compared to letting spine spit out many, many sheets of atlases and just use basic slot swap API?

It all depends. If you want to support modding, you will need to provide some dynamic loading mechanism to load the custom modified skins, unless you want the modder to overwrite your existing assets. If you want to support dynamic loading, you need to organize everything so that you know what assets need loading, one way would be to group atlases by skins. This would make things easy to add new skins as a single atlas texture package, e.g. to add a custom "robocop knight" skin.

The more important point however is that with unified origins you can easily yourself add skin packages as updates, without having to replace the main skeleton all the time. Having a unified origin for e.g. all weapons would mostly save a lot of additional work. Nevertheless, you can also write a custom offset script and store pivot offsets for each attachment and then offset the attachments programmatically if you prefer that.