my personal opinion only- they teach bad habits and can be dangerous. By default a retractable leash is designed for the dog to pull to get more room to move. This teaches the dog to pull when they want, not when you want them to. By teaching them heel with a regular leash, you are teaching them to respond to you. Also, had many run-ins with others that either don't care that their dog can reach another dog or have their leash malfunction and the lock doesn't work. This mean the other dog has all of the leash at their disposal. Personally, I never like to meet dogs on leash, it creates too many variables. I simply ask my dog to heel and keep walking.