That's certainly a very popular theory, the government smoke screen idea, and I wouldn't be surprised if they have used it to their advantage on more than a few occasions.

As for aliens and what they might or might not do I think it's impossible to say what their motives are because, if they do exist, we know nothing about them. We can't attribute human logic to some creature or life-form we know nothing about. I read a science fiction novel years ago where teenagers come to places like Earth to have a laugh at us; land their spaceship, get out, "spook the natives" then fly off again just for the fun of it. It's just as plausible as any other theory when we know nothing at all about practically everything in the universe, let alone the Solar System, Earth or even ourselves.