another explanation is that they make him feel more "human." i know a few stories (in the comics, mostly) have used the glasses as a symbol of his more human side, as opposed to the larger-than-life superhero.
one more likely possibility is that glasses are a sign of imperfection. it might help clark to feel more like he's fitting in, rather than having people look at him funny for being too athletic, seeing too clearly, etc etc.
i think that, combined with the idea that the glasses help remind him to keep his powers under control, is the best explanation.
Paul
