Ask for the old pads to be left in the trunk. (If the dealer is dishonest he will find some old worn pads and put then in there, but if he does that he is moving from "aggressive recommendation" to outright fraud, which could land him in jail. Unlikely a licenced BMW dealer would go that far.) The pads will tell you whether or not they really needed changing. But I agree with the SA: there is no point waiting for the brake warning light if the pads are obviously worn.