Agreed that it cannot prevent anything on it's own. But it is what gives us(the western world) the basis from which we can justify taking action. If the west takes strong and decisive action it will create a deterrent because the ones taking the actions will have to worry that they will be actioned in one manner or other.
Now, the west hasn't exactly been great at upholding these values which is part of the reason that we're where we're currently at.
It's not that vengeance can be bad, it is always bad without exception. Because the reason from whence it stems is not to mete out just punishment, it is from your own urge to cause greater harm to those that hurt you than they inflicted, it can therefore never be right to any reasonable person(distraught persons are rarely reasonable or rational).
No, I'm not saying that you'd be bad for having the emotion, emotions are part of being human, it's nothing we can turn off at our convenience. But acting on it will make you no better than the original perpetrator and no free society should ever endorse it, it should punish it without remorse whenever and wherever it manifests itself.
It is not ineffective because we as a state(or states) of law needs to have justification in taking action, we cannot just take action because we feel like it, not when the stakes are this high.
Therefore culpability is a way of gaining us that justification, if it can be proved that the crash was not due to an accident but was in fact because it was shot down with a SAM system then we can justify crushing whatever remains of Russia's economy with crippling sanctions, and even(if that is what we wish) to send in the cavalry to clean up the mess that is the rebels in eastern Ukraine.
I'm not advocating that there should be any trials, I'm advocating that we should use international law to justify our actions. There will be no courtrooms, no judges and no juries, but there will be states taking action(hopefully) and if we can establish guilt or culpability it is easier to justify the actions we take. If we can eliminate any reasonable doubt it will be easier to get other nations to rally behind sanctions and other necessary actions. Look at France for example who insists on continuing shipping their Mistral carriers to Russia, if we can eliminate the doubt that The rebels shot down the aircraft and that Russia is behind the rebels then they have to choose between stopping their shipment or being ostracized by the rest of Europe, as long as it's not clear where the guilt lies they can justify continuing their shipment by the fact that there's no evidence that Russia is behind all of this.
I never actually thought that Putin would actually take the way out, I was hoping against hope that he would though because it'd let the world deescalate this in a sensible manner, without it only passivity or escalation remains and personally I hope the west takes the latter option.
Putin already had his Anschluss, we've seen this happen before so lets not tread down the same old path again, this time we need to tell "Hitler" that this is enough, either you back here and now or we take you out like the rabid dog you are, not a step further. The last thing the world needs is another Neville Chamberlain pronouncing that he's created peace in our time. We can't keep pandering to the madman because he'll only take that as a blessing to keep going, we need to stop him before it gets out of control
And yes I know I just invoked Godwins law but for once the comparison is too accurate to ignore so please bear with me.