The main argument offered by the supporters of the war is to deny Al Qaeda a safe-haven from which to train and organise attacks on the West. Though terrorism can be organized in Oldham, Hamburg and Marseilles, Al Qaeda still believes it needs safe-havens in places like Afghanistan.

It's a weak argument. According to Stephen Biddle in Is it Worth It? in The American Interest:

The United States invaded Afghanistan in the first place to destroy the al-Qaeda safe haven there—actions clearly justified by the 9/11 attacks. But al-Qaeda is no longer based in Afghanistan, nor has it been since early 2002. By all accounts, bin Laden and his core operation are now based across the border in Pakistan’s Federally Administered Tribal Areas (FATA). The Taliban movement in Afghanistan is clearly linked with al-Qaeda and sympathetic to it, but there is little evidence of al-Qaeda infrastructure within Afghanistan today that could directly threaten the U.S. homeland. If the current Afghan government collapsed and were replaced with a neo-Taliban regime, or if the Taliban were able to secure political control over some major contiguous fraction of Afghan territory, then perhaps al-Qaeda could re-establish a real haven there.

He adds that the risk that al-Qaeda might succeed in doing this isn’t much different than the same happening in a wide range of weak states throughout the world, from Yemen to Somalia to Djibouti to Eritrea to Sudan to the Philippines to Uzbekistan, or even parts of Latin America or southern Africa.

And of course Iraq and Pakistan could soon host regimes willing to put the state’s resources behind al-Qaeda if their current leaderships collapse under pressure. Many of these countries, especially Iraq and Pakistan, could offer al-Qaeda better havens than Afghanistan ever did. Iraq and Pakistan are richer and far better connected to the outside world than technologically primitive, landlocked Afghanistan.

When the pro war crowd, or conservative commentariat, defend the war they appeal to the commonsense of the man in the street who supports the war against the elitist academics full of theory.

It's the standard anti-intellectualism of the Right that scorned argument, knowledge and expertise.