Is there any reason why a country should not say in order to maximise our income over a longer period we are going to reduce our output?

No, but the question is to what extent the reduced output is voluntary. Here are the actual EIA total liquids net export numbers for Saudi Arabia and my estimate for 2008:

2005: 9.1 mbpd
2006: 8.5
2007: 7.9
2008: 8.4*

*Estimated

The conventional wisdom is that almost all of this decline is voluntary. My contention is that most of the decline was involuntary. In any case, sometime next year, the cumulative shortfall between what Saudi Arabia would have exported at the 2005 rate of 9.1 mbpd and what they actually have exported will almost certainly exceed a billion barrels of oil.

So how much of a reduction would be needed to double the price? I would guess a reduction of less than half.