If restoring Constitutional checks and balances and showing that the rich and powerful aren't above the law, especially laws of basic human decency like the Geneva Convention, isn't reason enough, there are some very immediate national security reasons to do so, related to 9/11 and the Iraq War.
In the case of 9/11, that day George W. Bush said,
I have directed the full resources of our intelligence and law enforcement communities to find those responsible and to bring them to justice. We will make no distinction between the terrorists who committed these acts and those who harbor them.The Joint Congressional Inquiry into 9/11 found that the Saudi government helped the hijackers, and declassified FBI documents show a Saudi agent picked up two of the hijackers at LAX, set them up in an apartment in his own building, and funneled checks to them from the Saudi ambassador's wife.
Did the Bush administration use this information to punish Saudi Arabia or change the nature of our relationship with them in any way?
He even protected them by classifying the Saudi pages of the Joint Congressional Inquiry's report and tried to smooth over uproar caused by the sketchy details that did leak out.
He likewise said little to nothing about Saudi terrorists entering Iraq even though more came from there than any other country according to Israel, our Pentagon, and even the Saudis themselves.
Why didn't Bush do anything about this? Even if you don't believe the Bush administration was involved in 9/11, covering up the Saudi role is at least a bigger crime than Richard Nixon covering up a second rate burglary.
Worse, it means that we may be vulnerable to another terrorist attack because for all the Patriot Act bluster and trampling of our civil rights, the Bush administration did nothing punish or restrain the real perpetrators.
There is a similar issue with how we became involved in the Iraq War. What was once considered a conspiracy theory, that the Bush administration intentionally lied to get us into the war, is more or less accepted as fact by the mainstream media now.
However, if we don't prosecute those responsible, they are free to return to government at a later date, and do the same thing. That is exactly what Cheney and Rumsfeld did after lying about the nature of the Soviet threat in the 70s.
While the lies and liars from within the administration are pretty well documented, their helpers outside the administration, like those who forged the Niger document claiming that Saddam had tried to buy yellowcake, have not been outed and put out of business.
Perhaps most importantly, we have not had a public airing of WHY Bush bothered to trump up a war against Iraq and who it was meant to benefit. There are some clear clues like Cheney's secrecy about the energy task force he led that was pouring over maps Iraq's oilfields, and the Bush commissioned Hydrocarbon Law that would have given 88% of Iraq's oil income to foreign oil companies, a law that Iraqi legislators refused to pass even after being offered millions in bribes each by the oil companies. But those are just clues.
Without a definitive record of who lobbied for the war, who listened to them, and how they got their way, we are vulnerable to being misled into a war again in the future. If those who planned to profit from the war were punished, we would be even less likely to see it happen again.
It is a matter of public record that Bush diverted our attention from those responsible for 9/11 and fabricated a case for war, leaving us vulnerable to terrorist attacks from those he protected and squandering military resources we should have saved for real threats.
We are less safe because of it, and without the complete investigation and prosecution of those responsible, we will continue to be at risk.
If it does not happen, it would be because our government is looking after the interests of the very wealthy at the expense of the rest of us.