When people tell me, "all is empty, why bother?" I also point out dreams.
I say something like, "In a dream, nothing you see, hear, smell, taste, touch, or often apprehend(like memories, this is good or bad, what you think about things) is real, you know that, but you still suffer, you still fear, you still build up good or bad qualities. Suffering is real, fear is real, your good or bad qualities are real, so don't be foolish. If you can bare the results of your actions happily then do them, if not, act in accordance with karma."
This isn't true through and through, because it presents suffering, fear, good, bad, as solid and real, not dependent on conditions, but it shows careless people who deny karma what's wrong with their understanding of emptiness.