Well, the reason some wouldn't want there to be worldwide knowledge of aliens is simply the control factor. Once another race of people came to this world, human kind could start seeing themselves as humans rather than arabs, americans, britons, etc. They'd be terrible for governments, because the focus isn't on domestic (meaning Earth) affairs and on these aliens. All these precious political borders and gerrymandered voting districts would be rendered meaningless in the wake of contact with another planet. It'd take a big event to bring mankind together, and I think that alien visitation would be that event.
Governments can keep the important details secret. Operation Northwoods
, for example, was kept secret for around 30 years and there were no leaks until the project itself was declassified. When someone reveals top-secret or classified information, they don't make millions with a book deal. They're either intimidated into silence or arrested for treason.
Personally, I don't believe many abduction stories. Why would a group of beings travel to Earth just to impregnate women or probe men? Unless Douglas Adams was right about Teasers:
Ford Prefect wrote:Teasers are rich kids with nothing to do. They fly around the universe looking for planets that no one's made interstellar contact with and buzz them. [...] Yeah, they land in some isolated spot in front of some poor soul who no one's ever gonna believe and strut up and down in front of them wearing silly antennae on their heads and making 'Beep Beep' noises. Rather childish really.
Initiate discovery! Fire the Machines! Throw the switch Igor! THROW THE F***ING SWITCH!