I used to not see the point in extendable leashes at all, and everyone was saying how bad they are. But now I've found a use for them, within limited circumstances when other options less viable.
Where extendable leashes are NOT useful and in fact downright hazardous are in busy crowded places like city streets or busy pedestrian paths, unless you shorten and lock the leash so it's like a regular leash. (but I feel that a regular leash is more secure for these situations because the plastic handle can get pulled out of your hand more easily). otherwise, the dog can easily wander into the path of traffic or other pedestrians as the leash extends out, or get the cord wrapped around other people and things. This can be very dangerous. thus, I don't like to see dog owners walking their dogs on city or suburban sidewalks or on busy walking paths on these extendable leashes, because the dog can still bolt out into the path of traffic or other people.
But in areas where there is more space around you such as open fields, rural or semi-rural trails, parks (away from pedestrian paths) or other non-crowded areas, then it is safe to use an extendable leash and it allows the dog to have more freedom without being off leash.
Why wouldn't you want the dog off leash in those non-crowded areas, even if you have diligently trained a solid recall? In some places it's simply against the law. In other places, you may not have a clear view for potential hazards and thus not want to risk letting your dog off leash even if he has a good obedience skills. We train recall and other commands around distractions daily, but I don't want to be overconfident so whenever my dog is off leash I'm completely on my toes watching the environment constantly, and watching him constantly so I can know when to give him a recall or leave-it or stay command because he has a high prey drive and is very alert and fast. commands are only good if I can call him before he's gotten too deep into trouble! A couple of times I didn't see bicycles or other dogs approaching in time because of bushes or trees blocking my view or whatnot, and my dog had started running off towards them. His recall is pretty strong so in each time he came back immediately when I called him. But still, that momentary panic of seeing your dog take off towards other people, is not something I want to experience often even if I know that my dog will recall!! (My dog is a german shepherd so I know people will get freaked out seeing him running full speed towards them!) Therefore, if I have limited visibility around us, I choose to let him trail a long line, and if there's a lot of things on the ground get the line tangled, then my next choice is the extendable leash.
Then sometimes the ground conditions are yucky - like mud or other slush, and in winter a line trailing in the snow gets frozen solid and icy. So those are also times when the extendable leash works better than a trailing line for times I don't want to take risks being off leash.
Along the line of taking risks, if you are exercising a dog who has had little or no training or an unknown history - for example if you are fostering a dog from a shelter or taking care of someone else's dog - then one really shouldn't chance letting them off leash at all, yet they still need more exercise than just walking on a 6-foot leash. Again my first choice is a trailing line but if the ground conditions are yucky or would get the line tangled, then the extendable leash is useful.
That said, the only thing an extendable leash can do is to simply prevent the dog from getting more than 15 or 25 feet away from you. It doesn't allow you to "pull" him away from things easily should he be disregarding your commands. Thus I see it as only a safety net, not a control device.
Also, I highly recommend that the dog wear a body harness, and if possible attach a piece of bungee cord to it and then attach the extendable leash to the bungee cord. This is because if in a worst case scenario the dog suddenly bolts, he has 15 or 25 feet to build momentum before hitting the end of the leash, and this could result in serious injury if the leash is attached to his flat collar!! The bungee cord is to absorb some of the shock should the dog bolt and hit the end of the leash so it doesn't pull the plastic handle out of your hand or give a nasty jolt to you and the dog. I have found that this really does work. these are just backup precautions that I take and which I wish more dog owners who use these extendable leashes would also take. Hopefully with good training and sensible management the dog should not ever be runnning out to the full extent of the leash at full speed, but as we know the unexpected can happen so I believe in taking precautions.
Also, I find that the extendable leash doesn't affect my dog's loose leash walking. I think it could be because I seldom use the extendable leash (we are usually off leash, or trailing a drag line, or else walking on a regular short leash). Also I think dogs can and do differentiate different pieces of equipment and contexts - so if one is consistent and patient in training, the dog can learn when loose leash rules are in effect, and when it's OK to feel the tension from the extendable leash.