Most pedigree kittens you have to sign a contract which states they must be indoor only except if on a harness or in an enclosure, not just rare breeds, it's pretty much standard practice for purebred cats. Hybrids are rare and by law you need a licence from the council and are not allowed to let them roam, so harnesses are also useful for those. Mine never took to a harness, but I know plenty of people who let their cat take them for a walk. The idea isn't for them to walk to heel - you follow them while they explore stuff.