Whenever I hear "I don't need a leash, my dog is trained"...
Not using a leash is a sign that someone needs to check their ego and realize that it isn't all about them. Using a leash means I have a wide enough scope to recognize that the world is beyond my control, and that unanticipated events could happen at any moment.
I wear a seatbelt in the car because it's the law. I wear a seatbelt because I don't just drive on lonely country roads, and I realize there are other cars out there. Because I understand that, no matter how good a driver I may be, accidents can happen. Because I know that road conditions can abruptly change and I need to be prepared if something unexpected happens. Because I know that my elite driving skills can't always prevent shit from happening, and I want to be as safe as I can be if/when it does. Because I know that my wearing a seatbelt helps to protect other people around me, and in a collision I don't want my mangled body to become a projectile and harm those in the car with me.
I use a leash for almost the exact same reasons. It is the law. Unless I'm restricting myself to rural areas, there are other people and dogs and animals out there. Even if my dog is trained and/or friendly, not everyone else is willing or prepared to deal with him. I cannot anticipate what I am going to encounter when I leave my house, there could be distractions I never even dreamed of. There could be a pack of shrieking children, there could be a deranged raccoon, there could be a fucking earthquake. No matter what happens, I will still be attached to my dog.
Using a leash everywhere except designated off-leash areas tells the world: "I am a responsible dog owner, I respect the rules and the rights of other people to feel safe around my dog, because let's face it, a huge percentage of the dog is untrained with poor manners."
Not using a leash in areas where they are required tells the world: "My ego takes precedence over the safety of my dog, the law, and the rights of others. I don't care that my actions reflect poorly on other dog owners, or that the mere sight of my unleashed dog can lead to dogs being banned entirely from certain public places. I am a special snowflake."