I think this is nonsense, as surely vets will agree. How could a pulling dog possibly deform its movement simply by pulling against what is essentially a soft chest brace (as opposed to throttling itself by pulling hard on a collar, which CAN cause tracheal damage?)? Esepcially only during the time it is taken for walks? I have found that many show people are SURE all dogs, when put in a harness, pull and are untrainable. I think this is because few of them use harnesses and are very prejudiced against them and see them as not 'serious' enough -- but many who DO have also posted to lists on this issue before and say they always send their new puppies to new owners with harnesses. My own vets strongly recommend them for any small breed and the first time I brought Jaspar in, one noted with approval that he was on a harness. They feel this is much safer for all breeds of small dogs on walks than a collar for a range of reasons.
Where this pets column is right is in saying, take an obedience class so that your dog learns to walk politely and not pull. Whether this is done with the dog in collar or harness makes ABSOLUTELY no difference to the training, as my dog trainer and certified behaviouralist friends Lisa and Tara of Dog Training Ireland, who are on this board, have said to me before.
There's a much more pressing reason for cavaliers to wear harnesses and not collars for walks and that is becauser it is recommended by neurologists as a safety measure not to potentially aggravate a dog that may have syringomyelia, as noted above. If your dog still pulls -- then enroll in a good obedience class, and one where they will allow you to train in a harness.
Edel I can't guess why the lead clasp didn't work. I do know that soemtimes on any lead it is easy to not get it fully on the ring (on harness or collar) so that it is not firmly shut and maybe this happened in your case. I have two Puppia leads, each with a different style of clasp, and both are very sturdy and solid -- actually I prefer them to my other leads. Does the clasp look broken to you or not work properly when you examine it? If so, I'd return it, certainly. If it looks fine, then it may well be the case that it wasn't on fully. I am not sure how a clasp could open up unless it was totally broken or unless the little bit you push to open didn't then shut all the way -- with a brand new lead it could be a bit stiff til opened and closed a few times perhaps. Mine get sticky in cold weather if they've gotten damp.
Once, before I was using a harness all the time and Jaspar was about 6 months old, I clipped the lead accidentally to Jaspar's name tags and not his actual collar ring. The nametags were on a lightweight ring that expanded when he was pulling to go toward a fence at the park -- and he got off and disappeared into the park in the dark, with heavy traffic on the road behind. I was totally paralysed with fear. He eventually came back safely but ever since then, I check twice to make sure I have the lead safely clipped to the ring, as it is really easy to get it not quite right if you do it quickly.