My commute is fairly short (4 miles, one way) so if I just ride to work directly, there really isn't a huge fitness benefit. However, many (most?) of the days I ride to work, I'll use one of the legs for training purposes, such as doing intervals, or joining a group ride, or even just doing a long unstructured detour. That's the beauty of the bike, that it can achieve multiple purposes.
But to answer your question directly: I have no idea.