I believe that there are people who have seen unidentified flying objects which cannot be readily explained by documented natural phenomena or publicly known man made aerial vehicles. That being said, there is no good reason to attribute these flying objects to creatures from another galaxy. One might was well attribute them to angels, demons, elves from middle earth, or Morgrets from Mibbia under the sea.
It is an unrelenting compulsion of humankind to demand an explanation for everything, even if that means creating an explanation completely outside the bounds of reason. Why can't we, at least sometimes, just admit we don't know?
When someone asks me if I believe in aliens, all I can say is, "What's the difference?" What they are really asking is whether or not I believe that aliens are visiting our planet in stealth, abducting people, killing livestock, and conducting secret negotiations with the United States military-industrial complex. Now, if I choose to believe all these things, what am I to do? Unlike god based religious systems, ufology dogma offers no protection from alien abductions or livestock destruction. Of what use then is such belief? Show me how I might communicate with these beings (a repeatable experiment) and then I might be able to use the theory to some end.
Aside from being of no use, ufology lacks continuity. Why would beings with the technology to move from point to point in spacetime faster than the speed of light bother to kidnap us and look in our bums? They have interstellar trans-dimensional warp drives but they lack MRI technology and thus have to resort to an endoscope? Even if I do believe in aliens, I just can't believe this level of incongruity.