The thing is, the science is far more interesting because it's based on what we know at present. Personally, if alien life were like greys and the other menagerie of invented aliens human have come up with (especially those effin' Ewoks) then that would be a crushing disappointment - how drab and unimaginative.

But . . . just imagine if life on earth was seeded by a comet. If from those bacteria rose the the whole gamut of all the life that has ever existed or exists on this planet today. All of it in it's glorious diversity over deep time, from the critters of the Burgess Shale that don't fit into any known phyla to the dinosaurs to dung beetles, drongo's, dingo's and the countless, squirming legions of nematodes . . .

But think on. Cyanobacteria contain chloroplasts which enable photosynthesis, the driving force at the base of so many ecosystems that converts solar energy into material that enters the food chain, and if they seeded life on this planet then photosynthesis and chloroplasts did not originate on this planet - and that is an incredible thought that really transcends the mediocrity of modern UFO culture. The main powerhouse of life on earth is not terrestrial at all - it developed in a far off world a long time ago and is spreading around the universe over the eons. Can it be more exciting that that?