That's a fair long term view, and a sense for this type of nuanced choice is one of the marks of a good programmer. Not all students have the experience to judge the tradeoff, and so my goal as an educator is to default them to the safer of the two options. In this example, it's usually trivial to just add the static keyword to an already pure method. In my experience it's considerably harder to refactor a series of stateful methods that were incorrectly declared static, since you usually also have to plumb the state object through all of the callers, and their callers, and their callers, etc...
