How about this rule: no war? No? That's not going to work for everyone?
It is a purely pragmatic viewpoint: we know there's going to be war, whether we like it or not, so banning it is pointless. (Besides, we may want to go to war at some time and we'd like it to be legal to do so when that happens.)
It is also accepted that in a war people are going to get killed. This is a necessary consequence of war. If you couldn't kill enemy soldiers then essentially you couldn't wage war and we have already realized that you can't ban war outright. (People would just wage it anyway, it's too compelling.)
What is not however, or so it is thought, strictly necessary is for war to be overly cruel to the participants. Yes, you can get killed if you're in it, but people shouldn't be going out of their way to be cruel to you beyond this. There is usually a way to wage a war and achieve political objectives without torturing enemy soldiers and so nations can go to war without also at the same time being forced to ignore the rules of war, so long as all you are expecting them to do is not be overly cruel.
So the rules of war are what they are because that's more or less the most you can get away with and still have some hope that they will be adhered to.
There is also more than just a little colonial power bias in there, but that is something of a separate issue.
So it's fine to fill a guy with 40 holes, but you have to give him a chance to clot, or that's just mean.
Putting holes in people is just a very straightforward way of killing them or putting them out of action. Adding a non-clotting agent mostly seems like a purely cruel addition to this since someone who's been hit by a flechette is likely going to be out of action anyway. The non-clotting isn't needed for combat effectiveness.
Additionally, although this is seen from a completely different angle, it seems to me that the anti-clotting is ill advised from a military effectiveness viewpoint. What you want to do, ideally, is wound the enemy soldier so that he becomes an active burden on his side's evac and medical services. If he's dead he can just be left there for now and dealt with when there's available resources to remove bodies, but if he's wounded then manpower will need to be diverted immediately to deal with him which means less manpower to fight the battle that is going on there and then. (Even more ideally you probably want to maim him so that he becomes an immediate burden, and won't be returning to the action even after medical care; but pure maiming weapons are usually found in violation of the rules of war I believe.)