My typical response to the philosophical nay-sayers is somewhat like jgweeds. I argue that philosophy is just as much a search for truth and reason as it is a quest for wisdom. And, the world is full enough of people who "do" without reason or knowledge of truth.
It is usually just the ones who would say that philosophy doesn't make a difference to the "average joe". These are usually are political scientist or civil engineers and the like.
I liken philosophy to what the scarecrow said to Dorothy when she asked how he could talk if he didn't have a brain.
"I don't know... but then again, there are a lot of people with no brains who do an awful lot of talking!"
I'm not sure you are correct. I can't think of a single demonstrably true proposition that philosophy has come out with, I can think of some elucidatory sentences, maybe, but none that are definitively true and provable. Even if philosophers could show that we have no free will, or that inductive reasoning is unwarranted, I hardly think it would make a practical difference to the way we live our lives; and frankly, philosophers shouldn't care.
There are many works of philosophy, from grand works like Principia Mathematica
, to short articles like Strawson's Freedom and Resentment,
that have intrinsic value. The clarity of thought, power of argument, and analytical rigour in these works is the kind of thing that truly wows you by the time you finish them. It's not the truths that I think these works show, or the added understanding of the world they afford us, that gives them value for me, but the way in which they do it. If philosophers came to their knowledge in the same way that, say, biologists or geologists do, then philosophy would not be a fraction as beautiful, and I would take little pleasure in it. I can't say I see much value in continental philosophy, but modern analytic work, early modern stuff like Descartes, Hume and Locke, and Greek philosophy, contains a richness and level of thinking that shouldn't need to have a practical application to justify it.
If you want to be a philosopher you should reconcile yourself with the near certainty that you will never change the world, for better or for worse. However, you will get the chance to create something worth creating, a new Tractatus or Republic, perhaps.
You might want to read GH Hardy's A Mathematician's Apology
. It's a short essay that tries to justify the study of theoretical mathematics, but it is equally applicable to equally useless subjects like philosophy.