The more sophisticated argument from suffering against God's existence is surely its evidential form. It goes something like this:
1. If God exists, then there will not be any gratuitous sufferings. (Premise)
2. There are gratuitous sufferings. (Premise)
3. Therefore, God does not exist. (From 1 and 2)
Lest we forget, it's important to note that even if this argument were successful, it would not demonstrate that there is no God whatsoever. There very well may be a God qua creator and sustainer of all contingently existing entities, for example. Nevertheless, the argument from suffering, if cogent, does demonstrate that a maximally excellent God (omnipotent, omniscient and morally perfect) does not exist.
As theists, we have often challenged both premises of the argument from suffering. However, I want to tackle this at a different angle. I want to argue that in order for any non-trivial argument from suffering to succeed, the skeptic must accept, perhaps unwittingly, some variant of the Principle of Sufficient Reason (PSR).
The standard response to the logical version of the argument from suffering is that God may have morally sufficient reasons for permitting evil. The skeptic who agrees that the existence of God and the reality of suffering are compatible appropriately retorts that while God may have morally sufficient reasons for permitting suffering, he does not have such sufficient reasons in light of the many gratuitous sufferings.
Let that sink in for a moment. The proponent of the evidential argument from suffering agrees with the theist that God can and must have morally sufficient reasons for allowing suffering. It's at this point that the skeptic presupposes the PSR. If he weren't, then he wouldn't be requiring God to have any morally sufficient reasons at all.
I realize this is an ad hominem argument, but not all ad hominem arguments are fallacious or personal attacks. What I've been seeking to do in this post is simply lead the skeptic to the logically inescapable conclusion that either the PSR is true, or else God needn't provide any morally sufficient reasons for permitting suffering of any kind. The skeptic will simply have to pick his poison and either adopt some form of theism via the PSR, or else abandon the most promising argument against an omnipotent, omniscient and morally perfect being.