I think you misunderstood my speculation. The purpose is just the opposite.

The Saudis don't want to see the world economy, especially the US, go under, they need us for defense and they have substantial financial interests here. They also want to be able to maximize their profits by selling as much oil at as high a price as they can. They don't want to keep demand "as low as possible", and they certainly don't want to see Americans consuming less or conserving. But they also want to keep us completely dependent on them so they can influence our policy. A full petroleum reserve means we can operate independently of them for a little longer, in time of war or embargo, which reduces their hold on us.

I believe they struck a deal with Bush, they allowed prices to drop to help our, and the world, economy. It may also have been to boost American demand and help the Republican Party win the election. But the price they demanded for this favor was that we not let our reserve grow too much. Perhaps part of the price was that we buy oil for the reserves ONLY when prices are high, or maybe even not at all. This is consistent with the simultaneous drop in price and the Bush reluctance to purchase oil for the reserve at the low price.

It's a form of Saudi foreign aid, and like all foreign aid, it isn't about altruism. There is always an ulterior motive, and there are always strings attached.

The spice must flow.