What is better? Terrorists killing because of religion who are 150% army-ish and a country that is 10-30% army-ish, or terrorists that are 150% army-ish and a country that is 0% army-ish?
Part of the think is mutual assured destruction. A group of terrorist running into a Christmas party where 10% may be carrying guns - may think twice about starting to shoot. Of course there is the suicide part, but may only a few would have been killed before the terrorist were killed.
It is a practical measure. Give the option, I would rather be standing next to the guy that was armed.