It's my opinion that if you have a cat, then they need to stay on your property. I don't care if you let them out on a leash or walk them around the block in a child's stroller as long as they aren't killing the birds, getting into MY yard and crapping in my planters and grass. I love cats, but not when someone else's decides to use my yard as a litter box.
And anyway, why would you ever want to let your cat roaming? They might walk out the door and never be seen again. There are so many scenarios where they disappear. They can get hit by cars, taken in my strangers and made pets, grabbed by animal control and so much more. The biggest problem is that owners don't chip their cats or put collars on them, so if one of the scenarios I mentioned plays out, the cat will never have a chance to be returned by the humans who found it. Another problem is how can we tell the stray cats apart from the roaming cats? They might as well be feral strays the way their "owners" let them roam the city.
Cats are animals just like dogs. Would you let your dog roam onto other peoples' properties and into the streets so it could be slaughtered by some unsuspeciting driver who will always feel bad about the dog they hit? Would you let your bird roam around outside and trust it won't fly away? Would you let your rabbit out to roam in the front used and trust some animal like a bird or a cat won't catch it?
If you let your cat out roam, I think the biggest issue is that you're being an inconsiderate and bad neighbor.