Dog might be man's best friend but if you're trying to train your dog not to pull, the retractable leash is your worst enemy. Not only can retractable leashes snap, but dogs that get used to being walked on a retractable leash learn that the more they pull, the more likely their leash will extend. Dr. Becker, of Mercola.com's Healthy Pets even created a 10 Reasons NOT to Use a Retractable Leash List.
But, if retractable leashes are so terrible, why are they made? Well. For starters, retractable leashes started out with a noble purpose. To help train your dog. The idea was, you go out in your yard, or somewhere safe with your dog so you can train them the command "come," or "here," or "come hither" (if you're dog likes Shakespeare). Basically, you'd keep extending the length of the cord, little by little, until your dog learned to come to you on command.
The point of the retractable leash was to serve as a safety measure, so your dog couldn't run away during the training. As with many well meaning innovations, the original purpose becomes lost, and the innovation serves only as a convenience.
Sure, your dog probably loves being able to walk itself to the extent of its retractable leash, or RUN to the extent of it (especially on an icy day when it can catch you off guard and yank you to the ground with the momentum it builds), but if you're goal is to be able to enjoy the walk, then the retractable should be retracted. And, if you want to hear it from the master himself... check out this particular blog post by Cesar Millan on Cesar's Way. For convenient reference, the subhead in his article is "Don't use a variable length lead."