I kind of see your point, taks. But having a written document doesn't hurt. I mean I have clients who are well known to me, and so are all our expectations. But a written contract reminds everyone what those expectations are, and allows us to point at them in the event of a slide of standards.
On the subject of franc tireurs I'm no lawyer but reading the convention it seems to me that irregulars are protected even without being signatories provided they are acting in defence of their own territory (particularly in the event of an illegal invasion*). However, what is absolutely clear is that the irregulars must 'act within the laws of war'. This means they must accord rights of surrender, and not abuse captives. Given that Al Qaeda cells routinely abduct, torture, and murder captives for the purpose of home movies, it would seem they are voluntarily renouncing the protection of the convention.
In all, I'm beginning to think that the convention is actually very useful, in elucidating these points. What is obsolete is the public understanding of the convention, and matters military in general.
Incidentally I consider this all releveant as a critique of the AG's statements and opinions. What worries me now is who will GWB in his dingbat end of second term foist on the American people?
*I don't consider Iraq or Afghanistan illegal invasions, but many do, so the point is moot.