I think acne is a particularly interesting malady because it occurs mainly during adolescence, which is probably the most crucial time of one's life (if we're thinking about a paleolithic group of humans). As a young adolescent, your mother is no longer directly caring for you, and you don't have the strength or knowledge that you need to hunt or gather for yourself, so you need to make social alliances. Acne makes this much harder - in a paleolithic group, perhaps even close to impossible. I could even see it being so devastating to one's overall fitness (survival and reproduction) that it would be heavily selected against and disappear within a couple generations. But this is not the case; acne is still very much a part of many people's lives, so it must have had a gigantic benefit, one that would overcome the devastating shortage of social relationships at such a crucial stage of development.
There are two possible options that would make acne - and thus a lack of social relationships - a benefit. The first is that the lack of social relationships increases survival of kin by 'warning them away'. That is, by displaying spots, your body is telling others to stay away, or something bad might happen to them - they might pick up a disease, for example. The second option is that acne was a fleeting thing; it might happen here and there as a reaction to certain bad things, again a warning to others, but not a warning to 'stay away' - more a warning to not eat what you ate, or do the behavior that you did.
In neither case does acne occur chronically in a healthy individual. It simply wouldn't make sense if it did, and indeed we see in tribes all over the world that stick with traditional lifestyles a peculiar lack of acne, even during adolescence.
There's a possibility that modern circumstances are changing the conditions under which acne occurs; as in, we might not be sick, but because we're being exposed to a combination of chemicals or proteins in a particular way, our bodies think we are sick. There are a lot of novel chemicals and proteins (looking at you, GM foods) that could interact with our immensely complex immune systems in any number of ways. 90% of us have detectable levels of BPA in our blood, for example. Just think, for a moment, what other chemicals our bodies could hold, and how those chemicals are interacting, and then interacting again with our own enzymes. We don't have computers powerful enough to model the possible interactions, even if we had an algorithm. This is what's truly frightening.
Anyways, the theory that acne is meant to keep the opposite sex away (as if that's a good thing?) is trash, if humans weren't meant to have sex at puberty we wouldn't be fertile at puberty. Failure to deliver children safely is largely an outcome of terrible diets, epigenetics, and lack of selection that we've had for the last 12,000 years. (On a side note, apparently there was a study done in some hunter-gatherer tribe wherein they found that the females had a hormonal spike that prevented them from getting pregnant until 15 even if they had sex (and they did have sex). This is absent in industrialized nations, of course, maybe due to an interesting interplay between diet and xenoestrogens (such as BPA)?)