We are sociable creatures who enjoy telling stories. We learn and teach through parables and fables; it should come as no surprise, therefore, that we turn to anecdotes in laying out arguments for and against something.
The problem is, though, all too often we generalize inappropriately. We fail to see the larger picture, because we're interested in the compelling details in front of us that we can grasp directly. When we have a cold winter in Boone, it does not mean that global warming has stopped. The scope of the data needed to talk meaningfully about global warming is much larger than a season in the life of a single town in the mountains.
The same with education, with market policy, with vitamin supplements, and so on. The more grand the topic, the more important it is that we focus on the larger picture. For example, take health care. In the larger view of things, it does not matter if one patient experiences a three hour wait "because of socialized medicine" or another patient faces a jaw-droppingly unexpected million dollar bill "because the private insurers are greedy." What matters is how well the system works and at what cost.
Whether or not abstinence-only education works is a question that cannot be answered by pointing at Bristol Palin; it can only be answered by studies examining the changes in pregnancy rates and STD frequency in its wake (studies do find abstinence-only education lacking, as it so happens). Enjoy your stories, but before you draw your conclusions, how about holding out for science?