I agree with you. I think the complaints come down to "don't give people more confusing ways to write conditionals". And I agree with the complainers that it is easy to write conditionals in a confusing way, and that there are simply more confusing ways to write them when you can also use unless. But I think the problems come from not taking the time to make the conditional easy to parse, which is an issue whether or not you have unless. And it's not just trading if ! for unless (or if for unless !) -- sometimes it's "if valid" vs "unless !valid" vs "if !invalid" vs "unless invalid", depending whether it was easier to write the predicate for valid or for invalid. Even working in a language without unless, I frequently find myself encountering these difficulties, and I find that the only thing that helps is treating it like writing and deciding what logical progression will be the most helpful for the reader. What's most helpful for the reader won't always be to use unless, but sometimes it will, so it's nice to have the option.