Some thoughts from me, inspired / provoked by the responses so far:
Science doesn’t answer all the big questions - but it’s probably our best shot at finding out about our origins and whether we’re alone in the universe.
Science is not a panacea for all the world’s problems, but it’s our best hope for solving problems like climate change, AIDS and the food and water shortages that threaten the lives of millions.
Science is not necessarily more important than art, music or literature for fulfilling your potential as a human being, but it is essential if you want to contribute fully to a democratic society.
Science is not a collection of facts or truths, but it often provides the best approximations to these things (and allows you to identify falsehoods more easily).
Science is not an authoritarian conspiracy; it is a collaborative human endeavour, driven by creativity, need and ego.
Science is not good or evil, but it can be used for both. It doesn’t just show us the beauty of the world; it can reveal the full extent of its ugliness.
Science is not something you should be proud to be ignorant of; if you can quote Shakespeare or hum Mozart, you should know who Isaac Newton was and understand the Second Law of Thermodynamics.
Science is important.