I read an article today on Runnersworld.com. It basically illustrated the running habits of some of the fastest runners in the world and how those habits are quite contrarian to the mainstream thinking of pacing yourself through your training runs.
It was an interesting look at the mentality behind these insane efforts that these runners will put themselves through. Much of it is for money, since winning or placing in the money for much of these runners will change their quality of life immediately.
It is definitely worth the time to read, and maybe even try at your next group training session. Go all out and explode, if not, you might be settling for a slower PR. Link to the actual Runners World article: http://www.runnersworld.com/race-training/disadvantages-perfect-pacing