Here are three reasons why you should not use a retractable leash for walking your dog:
You have no control of your dog when he is walking ahead of you -attached to you by a plastic handle and a string. In order to regain control and get your dog back to you, you must either: walk/run towards your dog, clicking that plastic handle every step of the way, or mercifully start pulling at that string, snaring your hand in the process.
2. WHO IS WALKING WHO?
Have you ever noticed the number of dogs on flexi-leashes that lunge and bark at you and your dog as you pass by? That's no coincidence. Allowing your dog to walk 3+ feet ahead of you is essentially telling them that they are your leader, and they will 'lead' you in ways they see fit. This presents itself as your dog running ahead to pee on everything (No, they don't actually have to pee, they are 'marking' which means, placing small amounts of urine on things they feel belong to them - and further asserting themselves); and barking and lunging when they see fit.
Going back to reason #1, when you have to heel towards your dog to regain control, you are further communicating to your dog that they are your leader. This also causes them to un-learn recall (e.g. 'Come!').
3. DEFEATS INTENT OF WALK
Dogs are pack animals. Their natural instinct is to migrate (move forward) with their fellow pack members. If you trail behind them, you are communicating to them that you are the weakest member of the pack and they must either protect you or ignore you. A proper walk is a form of discipline for your dog. 'Discipline' in the dog world is not punishment, but rather a form of mental stimulation. It is also a means for you and your dog to build a relationship, and for them to gain trust and confidence in you as their leader, if done correctly.
Walking with your dog should leave you feeling peaceful and satisfied, not stressed-out or embarrassed.
A proper walk cannot be achieved with your dog leading the way, marking and sniffing everything, barking and losing control. This does not provide them with mental stimulation, but with bad habits that can lead to even worse behaviour.
Your dog does not need to pee during the walk (have them pee before you embark on your official walk).
A proper walk will stimulate your dog mentally and physically and teach them to trust you as their leader. Dogs with higher energy, such as a siberian husky or border collie, will require more 'physical' exercise on top of a proper walk.
When your dog trusts and respects you as their leader, they will not pull on the leash, lunge or bark. They will walk calmly beside you, and expect you to provide any direction and protection they may need.
Once you have mastered the 'pack walk', only then would it be OK for you to use a flexi-leash. Of course this is a trick, because at that point you will understand that the flexi-leash is unnecessary, because your dog does not benefit in any way by walking ahead of you.
A dog with boundaries and discipline is a happy, well-behaved dog who looks to you to learn more.
Keep moving forward!