The "harm" would be them having to answer a million questions and who among us are aliens. I think it would scare the hell outa some people. That is the point of soft disclosure and putting it in our face in movies, to desensitize as to not create a large scale scare. Then you have, "well why aren't using those methods for energy" yota yota, and now many industries and aerospace industries lose their grip. There are probably national security concerns regarding weapons and technologies that give us a competitive advantage and so on. Eventually though, if they really want humanity to progress, they will have to release it. Of course it's a distraction for everything else going on, but it's an important one to say the least and should be acknowledged, much more than a damn submarine that's for sure.