I think it's better if they're public. 

If you need a use case : to do calculations like chemistry or others, you sometimes just need to know how similar two things are (applyThreshold) or you want to check how dissimilar (diff). You don't necessarily always want to do it within an extraction. If you want to extract one thing from another and you believe they are approximately the same, so they should result in no remaining material, you would likely prefer to check the output of the diff or applyThreshold function to be sure before you perform the extraction, in order to handle exceptional cases where your assumption is wrong.