ET's who visit the Earth..hmm..good question! We know through our own development and programs like SETI, that WE are searching for other life in the galaxy..so why wouldn't some other race of persons be searching also?
Eventually, we will reach the stars. What will we find? I believe we will find life in abundance "out there". Not within our own solar system..with perhaps the acception of Europa, moon of Jupiter. There may be some primitive, aquatic life forms there.
But..in other star systems..of which there are "billions" (emphasis on the B, alah, Carl Sagan

), there is bound to be life.
Now, our only frame of reference is what WE do when we encounter life that is somewhat not as advanced as our own..past history shows we tend to corrupt it with our own ideas of how to live and our version of "god". Consider the conquistadors..and Christians who came to America and what they did to the natives already living here..sad, really..to corrupt a culture so they could be "saved".
Would the ET's have the same motivations we did/do? (even current events, from an American POV shows this same "attitude"..i.e. "lets bring democracy to the world!!)
Or, would they, being so much more advanced, having gained the ability to travel so far have some sort "prime directive" of noninterference?
Or..maybe they are just curious? Doing experiments, much as we do with lab animals?
I will say this though..in my explorations, I have found myself on more than one occasion on a planet I call the yellow planet. I don't know what its true name is, but everything seems "yellow", from the plants growing to the dirt, hence, my reference to it as "yellow." The inhabitants are not much different from us, but, I know its not Earth.