When you think about aliens visiting earth you have to get past the speed of light; it's not just a good idea, it's the law. Aliens would have to be long lived, be on generational ships, or have figured out a way around the light speed barrier. If the aliens lived in something close to earth gravity, they'd have to have a way to simulate that on their vessels. When you consider all the technical difficulties with space travel, it's reasonable to conclude that it's possible but unlikely that we have been visited by aliens.
There would have to be some big payoff to spending all those resources to visit the earth and it's probably more than the data from anal probes. Are there other forms of life in the universe? Almost certainly. However, the distance between planets containing life is, to say the least, a daunting barrier.