But it is terribly convenient.
For my own part I'm in Bill Hicks' camp. Why the Hell would they come thousands of light years and then come down to kick back in Fife Alabama or some such. (See the Relentless clip here.)
Then, I thought. Gee. If we postulated a civilization that had the ability to cross interstellar space, would we even know if they came? After all, it took us thirty four years and millions of dollars to get Voyager up to our own interstellar doorstep. Somebody who could zip down to us for a visit would have presumably much better technology than we do.
So, why is it dismissed so out of hand?
Part of it is, of course, the company the idea keeps. Most UFOlogists would have trouble playing Find Your Foot. But so what? The ideas held by fools need not necessarily be foolish. It wasn't so long ago that the idea of human space flight was considered foolish. And not so long before that the idea of humans being able to survive speeds of a normal train ride was in doubt.
So let's think about it.
Let's say we have a group of aliens, or their envoys, have come here to investigate us. They figured out we have life. They heard the incredible noise we produce. Like the Christmas poem, they've metaphorically sprang to the window to see what was the matter. They don't know us. So they want to check us out.
Granted it's a low probability. We have trouble conceiving the utility of visiting the moon. Anyone that might want to take decades to check us out is beyond our understanding. But we're talking aliens here and their motivation need not be ours.
So they get here. They hide for the same reasons biologists set up blinds: they want to study organisms in their natural habitat. Maybe they take samples. Maybe some of those samples are human. I'd hope they'd figure out that it's unethical to take human samples but since by getting here they've shown their motivations are not ours, it's unreasonable to think their ethics would be like ours either.
Why wouldn't that idea be worthy of investigation?
Imagine, if you will, a grant submitted to the NSF that says something to the effect, "to investigate if there exists possible evidence of earth visitation by extraterrestrials." Would you expect that to be treated fairly? Or better, would you expect it to not be laughed out of the committee?
I could even see possible avenues of research. Let's presume that aliens are bound by the known laws of physics. This means that to visit us they'd have to accelerate to a significant percentage of light speed and then decelerate. That takes energy. A lot of energy. A level of energy usage we can't even approach. So one area of research would be looking for evidence of such energy utilization. Indications of heat or velocity that is of astronomical in scope but local to our region.
Or we could examine transitory phenomena observed in space that doesn't fit what we know-- meteor tracks that don't follow expected trajectories.
We could scientifically evaluate abductees. If these people were abducted and examined, there might be contamination. Even if ET uses similar biology to ours-- DNA and the like-- it won't be identical. Perhaps they use right handed amino acids rather than left. Perhaps start and stop codons don't resemble ours.
I would expect failure of evidence since I don't think we've ever been visited. What bothers me is how we dismiss the idea out of hand without consideration. As if ideas were tainted by those that handled them, housed in a sick bed and ignored ever after.