Sure, the illusion that we have a free will is practical. Nobody is denying that, just like nobody is denying that the illusion of seeing your hand as a solid object serves a purpose. There are all kinds of perspective-related geometrical illusions, for example, our minds see all the time, but nobody would ever say that once you are on an airplane, the cars are the size of ants. They look like that, but we know it’s only an illusion to help us measure the distance.
The illusion of free will can then help us to find who is culpable and responsible. I think we all understand why and how this works, but I see no reason to call this illusion any more real than that of seeing cars as being the size of ants and saying they really are that small.
You forget there are two aspects of free will. By denying one you also deny the other.
A determinist sits in the restaurant with the menu card. Say, of the 20 dishes on the menu card he likes 3. Realising that one of the dishes is the same as what he ate yesterday, and that the other might be too much, he choose for one of the dishes.
Now what does it mean that he chose for this dish? It means that his choice was caused by him. It however does not mean that he himself was not caused. Looking from his inner experience, it is clear for him that, given that nobody coerces him to take a dish he does not like, that what he chooses depends only on him. So was his choice determined? Yes, it was. Did his choice (causally!) depend on his wishes and beliefs? Yes, it did. So is he correct that his choice depends on him? Yes, it is.
Does it mean that his choice originates in him and him alone? No, of course not. Now did the idea that we are determined change anything about his choice? No, not at all.
So there are two ‘experiences of free will’: the fact that what I do next depends on my wishes and beliefs. That is free will in the compatibilist sense. And there is the experience that what I do arises from nowhere out of me, i.e. it has no causal history. That is an illusion, the illusion of libertarian free will.
In your sense, George, you cannot even say that the ball flying to the window is the cause of the breaking of the glass, because the ball was kicked by a boy. But the boy was not the cause either, because it was his friend that asked him to go playing football, etc etc. In this way nothing is a cause, because everything is caused. In the same way you deny the causal efficacy of wishes and beliefs: but in fact they are just a part of the causal network of events.