I have thought of this subject quite a lot. Yes I would come into earths atmosphere for samples and surveillance purposes. Despite the fact we are a hideous excuse for a civilized species and life isn't really all that rare throughout the universe life is still rare enough and special enough to warrant further study. Think about it. They live the life of gods. Travel throughout space, time, and god knows what else cataloging every planet that contains life. Masters of the universe. They are only limited by our perception. I believe many of these craft/beings hail from other worlds and or universes. The thing that gets me is how we are still being visited with such frequency in all corners of the globe. It indicates a few things. A. There are an unprecedented amount of advanced civilzations(type III and IIII) permeating the universe as I suspect. B. we have definitely either been discovered or announced our existence the cosmos. Granted our space brethren have been visiting since our species was first created. It's just mind boggling to realize each separate sighting could be a craft from a different civ. do the math.