Of all the non-terrestrial theories that have been offered to explain the UFO phenomenon, the extraterrestrial hypothesis (ETH) has always seemed the most plausible one to me. I don't think it's been proved, but I think it's a better bet than the others on offer when one looks at the evidence, and the science.
The evidence seems to indicate that at least some UFO cases represent a non-human intelligence at work.
The science now tells us that there are almost certainly other intelligent beings in the galaxy, and if they are more advanced than us, there's a reasonably good chance that they could make their way here.
So, you might ask, why am I always so hard on the ETH supporters amongst ufologists?
First, I think it's important to remember that the key letter in ETH is the "H" - it's still just a hypothesis, and anyone who tells you that they can prove that aliens have visited Earth beyond a reasonable doubt, or even on the balance of probabilities, is putting the cart well before the horse.
Beyond that, however, I think the biggest problem with the ETH supporters within ufology is that they're so... "limited" in their outlook.
On the one hand, they are convinced that aliens have visited Earth, and in many cases they are convinced that they are still visiting Earth, and interacting with humans in all sorts of ways, some good and some bad. They are of the "nuts and bolts" school of thought, i.e. Joe Alien made his way to Earth in a flying saucer, in much the same way that Captain Kirk and all of our other science fiction icons make their way about the galaxy.
This is what I call "Keyhoe-ian" ufology, because it is based directly on the way of thinking that Major Donald Keyhoe first put forward in the 1950s. It is out-of-date, and badly out-of-touch with modern science. It presumes that aliens are only a few hundred years or so more advanced than us, which is highly unlikely. It presumes that the aliens are preoccupied with us, and that we are somehow important to them, which is also highly unlikely. In short, it is a point of view that is based on what people who grew up in the pioneering days of sci-fi and the space race expect of their aliens, and not the point-of-view that modern physicists and astrobiologists take.
If aliens are here, it is probable that they are far more advanced than we are, by an order of thousands of years, not hundreds. As Michio Kaku stated in the video I posted here earlier today, we would be to them as ants are to us - beneath their notice. This would explain the inherent weirdness of many UFO sightings - things that appear to us almost as magic, or something that in a different era would have been framed in religious terms. As Kaku noted, there may well be a galactic conversation going on, but in a "language" that we are thousands of years from being able to truly comprehend.
The pro-ETH stance of people like Keyhoe, and his successors, like Stan Friedman, is a relic of a different time and place, which is ironic when one considers that these people often criticize scientists for not being open-minded about the UFO phenomenon, and for being stuck in the past.
The ETH supporters may well have it right about aliens being here... but they are almost certainly wrong when it comes to the how, and why, of that presence. If you doubt that conclusion, ask yourself this question - when it comes time for humanity to travel to the stars (as opposed to the Moon or Mars), many hundreds or thousands of years hence, what will we look like as a species, and how would we view another species that we may find on some distant planet that is at our current level of technology? Does anyone really think we would care, anymore than we care about those ants crawling around in our backyard?
By focusing on the idea that little green / grey men have been coming here in nuts and bolts spaceships, ETH supporters like Keyhoe have done a grave disservice to the search for truth about the UFO phenomenon, and its possible alien origins, in the same way that thousands of years of religious leaders have undermined the search for the true nature of God by force-fitting it into a limited paradigm that simply served to reinforce their own worldview. They have not sought wisdom, nor understanding - they have simply proclaimed an "answer" which has been no answer at all.
If the truth behind the UFO phenomenon is that at least some cases represent an alien intelligence, then like God that truth is probably beyond our comprehension, at least at this time in our development. This makes things more exciting, because it shows us a direction to the future, and a reason for getting there someday.
The reductionist approach that has been adopted by the nuts-and-bolts crowd, on the other hand, which seeks to make potential alien life over unto our own image, lacks vision. It is more concerned with what they see as the destination, and their need to get there now, when what we should really be focusing on is the journey, and the wonders we may discover along the way.
That's the real signal in all of this. Everything else is just noise.