[hansef]

In an ideal world with infinite time for arguing with... well, I'll avoid an ad-hominem when responding to an argument about ad-hominem attacks, but... in an ideal world it would be worthwhile to drill down through the onion layers of rationality, irrationality and emotion which drive any argument we disagree with, every time.

Sometimes this spelunking is both constructive and instructive.

Sometimes though, dismissing the assertion that the earth was created 6000 years ago or that the world will be ending next month with a quantitative assessment of the intellectual capacity of the asserter is a completely appropriate shorthand.

The key is judicious application. ;)