I understand that some people, who no doubt have very placid cats, and who have spent time getting them used to the things, and who don't take them further than the garden, can find them useful.
But ...
This week alone we have had three cases of harnesses causing serious harm. The first - someone decided to take their cat for a walk in the park. The cat got spooked, flung itself around, broke free of the harness and promptly disappeared down some sort of fox hole. We've been staking out the park, we've had motion activated, night vision cameras down the hole, and nothing. It's been a week now. The cat is nowhere to be found.
The second - a man had a female cat in a flat. Unneutered due to the terrible shortage of vets in London, he was on the waiting list for her to be done. Pets at Home had some sort of promotion/event thing about indoor cats, and how all cats needed to have outdoor access, and here are some harnesses etc etc. So he was persuaded. He put her in one, took her outside and, just like many cats do, she totally freaked out and managed to escape the harness. One of our rescue workers had to spend 9 hours in full sun in sky high temperatures yesterday trying to trap her, after the owner had been unsuccessful in getting her back for around 10 days. She was caught, but could very easily not have been.
Third one - similar story to the second. This time a male and still missing.
Previously we've had calls where the same thing has happened - at a motorway services!
Harnesses are not safe. Most cats hate them. Think very, very carefully before using them.