My Christian Scientist grandmother would have disagreed with your medicine example, and in fact chose to die rather than seek conventional treatment for an illness.
But on behalf of your example, I should add that when I give talks to groups about gardening, I always start with the 'what plants need' outline (regardless of the age of the audience; it seems to work). And then I usually add, 'and sometimes they need protection--from the weather, from pests, etc.' I usually get a laugh by adding 'and from us, sometimes!'
These concepts of absolute/needs vs. relative/preferences from REBT are actually useful to apply to life in many ways, and using the more extreme examples, as we have done here, simply illustrates how far they can go. Albert Ellis wrote an interesting essay after 9/11, for example, which I will try to find. It also can help us to have more civil debates when values conflict. In our country there has been debate since 9/11 about how to balance the values of security vs. freedom. On the other hand, when we watched as thousands of people crammed into the convention center in New Orleans after Hurricane Katrina, running out of food and water (and medicine), we realized what true needs are.
But Marte really summarized the way to use this: "I only change the 'should's' and all the other absolute thought indicators when the thought coming with it will bother me/do me harm. Other than that I'm not gonna change my language."
If we understand that need is often an exaggeration, if we know that we are using the language with some acceptable imprecision, then we can focus on the core principle: that we create our own distress, and that often it results from turning a preference into a need.