Try and rationalize it away as long as you can.
It helps some folks sleep at night.
Rationalize WHAT away? Look, I am not suggesting that there is no other life in the entire universe. I AM suggesting that Mitchell's claims that earth has been visited by aliens, that government scientists/agencies have been examining alien bodies in secret for 40 years, etc., are a bit far-fetched.
Let's assume that there IS other life elsewhere in the universe. Personally, I consider it quite likely that there is. Now, consider that radio signals only started being broadcast on Earth in 1901 - 107 years ago.
This means that nothing more than 107 light-years from Earth could even know that there's anything or anyone here. And that assumes that the radio signal could possibly be picked up that far away. But let's assume that by some incredible quirk of fate, an alien civilization a mere 50 light-years away picked up the signal (keeping in mind that our galaxy is some 100,000 light-years across, making such proximity remarkable, to say the least). Even IF they could travel at the speed of light, and even IF they left their planet the instant they picked up the radio signal from Earth, and even IF they could travel at or near the speed of light, they would only have arrived here LAST YEAR.
In order for aliens to have visited earth in the 40's, 50's, or 60's, not only would they have had to be very close to earth to begin with, they would also likely have to be capable of traveling many times the speed of light.
Now, you can throw around theories about worm holes, higher dimensions than the 4 we all know, or aliens on Mars, or whatever, but I find it astonishingly unlikely that aliens have been visiting earth.
This is not because I refuse to believe they exist (no one knows one way or the other), it is just that it is so incredibly unlikely that any are close enough to earth to have become aware of our existence (a sphere of 107 light-years from earth, relative to the size of our galaxy alone, is pretty tiny), much less to have gotten here.
We've been listening and searching for signals in space for a couple of decades now, and we've found zilch. Is this because there are no other civilizations in all of the universe? Not necessarily. It is just as likely that none of them have been around long enough, broadcasting signals strong enough, and from close enough, for us to even be aware of their existence.
The point is that the universe is a pretty big place, and the odds of two civilizations ever coming into contact with one another are incredibly remote. Equally remote are the odds that aliens have been visiting earth for decades, abducting people, crashing and leaving bodies behind for shadowy government agencies to examine, etc., without any solid proof ever coming to light. It's ridiculous.
Now, why don't you try to rationalize a strong belief that aliens HAVE been here? That I'd like to see. Actually, no I wouldn't. I've already read and heard plenty on the subject, and none of it has been any more convincing (or supported by facts and/or logic) than Mitchell's claims.