Lets assume for a moment that Christianity must operate on said principle, what would this mean for the problem of natural evil? First of all, one must consider what would be considered harmful and beneficial on Christianity. Obviously, one would have to place spiritual concerns above physical concerns as any damage to one soul would be permanent and much more weighty (leading one to either the destinations of heaven or hell), barring intervention from God, whereas physical harm would be temporal and much more reparable (through medicine, surgery etc...). Because of this physical sufferings which bring about spiritual benefit would be reasonably considered good under a Christian worldview. Even with these considerations alone, we see a theodicy forming in response to the natural problem of evil. It could be supposed that all natural suffering preforms some function which benefits the spirit of an individual, and thus would be considered good. Granted significant objections could be raised against such a theodicy. For example, what about those who become embittered because of there suffering and grow to despise all that is good? Given such objections, it seems reasonable only to regard such a theodicy as a partial theodicy and not a complete theodicy against the natural problem of evil.
But what considerations should we take into account in regards to the principle that whatever harms is evil and whatever benefits is good? One more basic consideration would certainly be that given a Christian worldview, such a principle would, at the very least, be incomplete. For one thing, such a principle would not, by itself, allow for the concept of justice. Justice would require punishing harm with harm and benefit with benefit. Justice, being essential to Christian ethics, must be taken into consideration when discussing the problem of natural evil from a Christian perspective. Indeed, given justice, at least some forms of suffering may be due to God justly punishing those who have committed wrongs. Again this would only be a partial theodicy as making it a complete theodicy would lead to the conclusion that innocent children and even the unborn are deserving of suffering, and furthermore, that every suffering we do endure is an indication of some sin we've committed. Nevertheless, some suffering does seem to come about as a consequence of our actions (global warming may be one such example).
There are perhaps dozens more considerations to take into account when analyzing the problem of natural evil within a Christian worldview, but this at least gives a brief introduction into how I have dealt with such a challenge to the Christian worldview.