I suspect you and I agree on this, but I'd like to reframe your argument a bit. I would argue that a good manager should strive to never explicitly say "no". There are plenty of ways to limit misguided ideas without explicitly saying no. For instance, you can say "Yes but...", "Perhaps, but only if...", or (my personal favorite) "Good idea, but I personally like your other idea much better". The advantage this approach has is that it makes sure that you're not shutting down a good idea that has some major (albeit fixable) flaws.

Of course, sometimes you have to just set your foot down and just say "No". But I've noticed that as managers get more experienced, they tend to rely on their ability to do so less and less.