It is possible to use a special stone to prepare pizza in your barbecue, but it is not easy to get the bottom and top heat in the right balance. The high lower temperature (plus 250°C.) can be achieved, but the upper temperature needs more time. I am therefore more inclined to use that stone for baking delicious flatbreads that serve as an edible plate for all those other tasty things that go on the fire, or, simply torn into pieces, act as a base for salads and spreads. What about an Italian focaccia, an ancient bread baked by the Etruscans as early as the 5th century B.C. in a way that has not changed at all since then and therefore bears strong resemblance to your barbecue (with that stone in addition)? That's different than some cowardly vacuum-packed baguette that has to be baked in the oven. Not to mention the smell that a fresh home-baked bread spreads. In short, such a stone is not a crazy investment.