Why do the USD and GBP have value?  Ask your audience to explain that to you... after all, they clearly believe that those currencies *do* have value.  They'll probably tie themselves in knots but it's not hard to help them see that a currency doesn't have to be "backed" by something to have value in the real world...  so once you've established that things aren't at all clear in the "real" world, you only then have to establish that Bitcoin is at least as good as what we already have.