I have seen them of course. Are you absolutely certain that some of those are definitely extraterrestrial? I am not, I need something just a bit more substantial, not quite there yet 
Apart from technological documentation like photos, I think a sort of psychological contagion can happen, lots of people "see" the same kind of craft because they have been influenced by others. There may also be chemical changes in the brain, like with certain drugs, that produce visions of the same certain kind of beings, some kind of animal or plant or craft or phenomena. Whether these are "real" is the question.
I think the impact would be world changing, but remains to be seen. In fact, conspiracy theorists claim governments know about the aliens and have the crafts and bodies but every government has gone completely silent on it because they are afraid of world wide panic. The people would demand a military response and so on. For what other reason would they stay silent on it?
The "there will be panic" argument imo is nonsense. Probably pushed by very religious people within the DoD and intelligence community.
More credible reasons for the silence are the fact that they have been working illegally behind the backs of Congress and many Presidents. Violates the American constitution. The people and companies involved don't want to go to jail.
And NORAD / USAF having to admit they have zero control over North-American airspace is humitiating to them. Non human tech also makes NASA look like a boyscout organisation. Same for US Navy. Especially as those organisation get around 900 billion a year from the US taxpayer.
Also there seems to be a sort of arms race between the USA, Russia and China with regards to reverse engineering attempts of non human advanced tech.
Whoever succeeds first is a clear winner. And has a huge advantage over the others.