I have to admit that I'm not always quick to praise an RW article but this one nails the topic on a few points.
First, the point that many runners tend to race their workouts or set goal times for their workouts. The best thing these people can do is to drop that mentality.
Second, the point I make often of learning to listen to your body in workouts. You can fine tune your workout to how you feel on the day. Running by splits, if you're having a bad day, you could bury yourself. If you're having a good day, you might not get the most you can out of your workout.
Third, related to the second point, listening to your body in races. If you know how to listen to your body, doing so can be more effective and offer better real time feedback than taking splits.