The problem with the term "need" is that it suggests there are no substitutes for the item in question. Thus, people will pay any price for it; however, the law of demand says that at some price, people will take less of something, including none of it. In response, a student might say, "Diabetics can't do without insulin" or "People can't do without food." I say, "Yes, they can; diabetics have been doing without insulin for thousands of years." In some poor African countries, people do without food. Of course, the results of doing without insulin or food are indeed unpleasant, but the fact that the results are unpleasant doesn't require us to deny that non-consumption is a substitute for consumption. Again, I tell my students not to purge their vocabulary of crying, dying and urgent needs; just don't trick yourself while you're tricking others.
You say, "Williams, it doesn't sound like economics is a very compassionate science." You're right, but neither is physics, chemistry or biology. However, if we wish to be compassionate with our fellow man, we must learn to engage in dispassionate analysis. In other words, thinking with our hearts, rather than our brains, is a surefire method to hurt those whom we wish to help.