It is hard to rule out anything for something that can not be explained. Why not fairies, witches, dragons, superman, krypto, the batwing (batman's plane), ironman, etc.etc.etc? All are just as valid an explanation as time travel and ET. None have been shown to exist but that does not mean they don't exist. I prefer more mundane explanations for UFO cases. However, if a case can not be satisfactorily explained, it does not mean it is time travelers or ET or whatever. It could very easily be simple misperception by the witness that they did not accurately report. This is a known fact that witnesses can make mistakes. So, we are left with what you want to accept as likely. Again, I prefer things that seem reasonable and are known to exist. In the words of Dr. Richard Feynman:

"I think that it is much more likely that the reports of flying saucers are the result of the known irrational characteristics of terrestrial intelligence rather than the unknown rational efforts of extraterrestrial intelligence."