The main reason for recommending against making changes inside the threekit folder is to make the act of updating the dev-kit for that project easy and less error prone. If you feel the dev-kit mostly has what you need for the project and that updating will not be likely then making changes to the threekit folder is probably the simplest and best way to proceed. I agree its not worth its own component.
I do recommend posting about the changes or features you find yourself implementing in the threekit folder as quite often it ends up being something that we would want to add to the dev-kit as part of its next release and make available on all subsequent projects. This is a good example of that - the ability to show/not-show the number of options present in a dropdown.
