Let's suppose for a moment that an omnipotent, omniscient deity exists. Your argument implies that there is nothing [this] God could possibly do which would justify us doubting his benevolence. If he turned Earth into Hell and tortured us for all eternity, you could answer "he might have a reason, you can't know cos you're not omniscient like he is". Really. That's just absurd.
One could also turn it around to defend the claim that God is omni-malevolent, or perfectly evil. This strikes us as silly, of course - just look at all the good in the world! - but if the 'ignorance' response is sound then identical logic can be used to negate all such evidence. Sure, it seems to us that the world contains goodness, and it's hard to see why a perfectly evil agent would want to allow this. But we can't be sure that an omniscient evil-doer couldn't have some hidden reason to allow these goods. Perhaps it's all part of their plan, and serves to bring about some 'greater evil' which is beyond our comprehension.
So I think the biggest problem with the ignorance response is that it makes us too ignorant. Such extreme skepticism just seems entirely unwarranted. It's obvious that the world is too good to take any suggestion of an evil-God seriously. Similarly, the world is not good enough for an omni-benevolent God.
This then leads to Macht's intriguing suggestion that God simply isn't omnibenevolent:
God is good, but he is only good to us in the ways he promises to be good to us. And these promises don't include anything like "maximizing the happiness of all people" or "making sure no bad things happen to anybody, ever."
Put like that, he doesn't really sound any better than any averagely-decent, promise-keeping human being. And of course if one takes the Bible literally (*shudder*) then he sounds even worse than your average person. But I guess this formulation could be tightened up so that God is still impressively good, yet not 'omnibenevolent'. I'd consider that a respectable solution to the problem of evil.
Even better, I think, would be to deny omnipotence. I actually find the idea of a benevolent but powerless deity rather appealing - it would just add so much more meaning to our lives if God actually needed us. Our choices would actually matter in the 'fight against evil' (I mean this in the fantasy-novel sense, not the neo-conservative sense). If God is both omniscient and omnipotent, by contrast, then "all the world's a stage", for surely nothing we do would be allowed to get in the way of his ultimate plan.
Further, as mentioned in the earlier post, if we allow an omnipotent, omniscient, omnibenevolent God on the basis of the 'ignorance' defence, it seems to rob us of any moral responsibility. Such a God would ensure that things turned out for the best, so there'd be no point in us meddling. If God wanted a tsunami-victim saved, he could easily do so himself. The fact that he didn't would seem to imply that harming the victim is all part of his divine 'plan'. So why should we interfere with that? Why should we help others if God doesn't - wouldn't it be hubris to presume to know better than him?
Come to think of it, one might plausibly use this line of thinking to explain why God must never interfere with the world. Human moral responsibility is surely a very important good (right up there with the old favourite of 'free will'). Yet it seems the only way to protect this is to ensure that we believe that God won't prevent evils, and that it is instead up to us to do so (to the best of our ability). The main problem with this response is that only the belief is necessary - it need not be true. So God should secretly prevent evils whenever he can do so without our noticing.
Alternatively, one could deny God omniscience (and omni-benevolence with it), as Raymond Feist does in his fantasy novels, if memory serves (I read them many years ago). The idea, then, is that it's up to us to teach God, to show him what 'works' and what doesn't. I find this by far the most appealing religious picture I've ever come across. Imagine the responsibility! It's up to us to ensure that God turns out well, that he learns right from wrong, and so forth. Wouldn't it be grand! (I suppose I could just have kids of my own, but I don't imagine any of them would turn out to be omnipotent.)
Lastly, I suppose, we could dispense with wishful thinking altogether and just admit that we're the only 'intelligent designers' playing in this corner of the cosmic ballpark. This is not nearly so fun as the previous suggestion, I admit, but it has the added advantage of being true, which may count for something. [Begging the what? I don't see no question. Shush, you.]
Update: Macht responds here.