I used to have an older, sedate dog that I'd rescued. She and I would walk everywhere off leash, even though she was a large dog. She never strayed more than a couple of feet from me and if another dog passed, she'd calmly wag her tail from a distance.

I now have a young, active dog. I rescued him at 6 months and initially I thought I could teach him to walk with me the way my old dog used to. Well, I will be able to eventually, but right now - he is always on a leash unless we are in a designated off-leash area such as a dog park. Not because HE is aggressive, he isn't. He's super friendly. But, as you pointed out - what if the other dog doesn't want to be friends?

Everyone needs to be able to politely share public spaces. that includes dogs. So... yes. My dog is currently walked on a leash.