Sometimes it should be; it depends on evidence. Some people say UFOs are supernatural or from the center of the earth or our future. Such notions fly in the face of science and rationality; the ETH doesn't. Where do UFOs usually appear to go after a sighting? Up, which says something.
Yes, but the high strangeness factor involved in UFO sightings seem to suggest that they are half way between the subjective and the objective, physical manifestations of some description (which may even be Extraterrestrial in origin) which both play on our deep expectations of the unknown, while subverting them at the same time. It seems that when they are flying in the sky, they are unidentified flying objects which defy many known laws of physics. But, when one of these things land, well that's when things get really weird, perhaps suggesting some sort of wildly different type of intelligence, trying perhaps to find a middle ground. Or, perhaps they are an intelligence native of the earth. I wouldn't say that the ETH is completely out the window by any stretch - just that it is infinitly more complex than the very basic ETH scenario.
What makes me a little bit weary of the ETH (just to play Devil's advocate) is that, in the middle of the nineteenth century, we became aware of the possibility of space travel, and of life on other worlds. This was the myth for a secular age, and as a myth it had everything - utopian hope, dystopian fear, and it harkened to the future, to scientific progress. And people began to see strange things in the sky. Now, it could be argued that people have always seen such things. And perhaps the phenonemon has always been the same strange thing. But here's my point. We have this myth, and it seems to fit the facts on a surface level. But when you go a little below the surface, at some of the other things people have reported, the whole picture rather than the particular, it seems something much more bizzarre and subjective is going on. And this is one of the reasons science cannot yet cope with the topic - because it confounds and distorts at every turn. And this may be in the nature of the phenonemon.
People have reported fairies in the past, and all sorts of demons. The native americans talk of the trickster, which both plays the fool and also is one of the wisest of them all. Think of the jester in a Shakespeare play - always the one who can see what is really going on. Think of Loki, who at times seems to be one with Odin, the highest god in the Norse pantheon. These beings are never purely malice, though they do produce fear in us - a fear of a thing which subverts the natural order. Carl Jung says that the thing we most fear about the archetype of the trickster is that it is unconscious and unknown, lying beyond the thing which we call conscious perception. For Jung, when such things pertrude upon reality it is called synchronicity. Jung felt that they were manifestations of the mind, but minds in interaction (unconsciously) with every other mind, and perhaps even with every other form of consciousness on some level. This is a quite controversial theory, but, again, who knows.
I personally think that people have been seeing such strange things from time immemorial, and it may be something inexplicably and intimately linked with intelligence. In a sense, they may be the gods of old, the ones who dared us to take the bite from the apple, or tempted pandora to look in that box. Pure symbol, essentially, a symbol of the fringe, the unknown, of the boundaries of knowledge, and a teaser of what's to come.
Besides all of that, many have pointed to the initiation of the shaman and the abduction experience as having similar themes running through them. A lot of these rituals, after which shamans were reported to have been able to do amazing things, and they were using drugs to enter into these states, to communicate with the earth and their gods - symbols for this or that.
These symbols could be more like a framework for something deeper that is going on, like a type of wisdom being imparted. And perhaps the UFO phenonemon is modern version of this.
I am brought back to that quote, not sure by who (I heard someone use it on the show), "UFOs are here to make us think."
If you are sceptical about just how weird these things can get, check this out http://www.scribd.com/doc/4452389/Dimensions-A-Casebook-of-Alien-Contact
Jacques Vallee, IMO, is the authority on this subject.