Well it's good to see A. you apparently already know what an alien would and wouldn't want with us, and B. You completely missed his point. As we don't understand the alien psyche (assuming they even have one) we can't make an educated guess on what they would/wouldnt want with us. More to the point all he's saying is that we should be careful and maybe shouldn't go willy nilly announcing ourselves to the universe. Does that make sense? Can you see the logic in it or are you still under the impression that E.T. just wants our Reeses Pieces?
And if they're significantly more advanced than us, it would be like a roach trying to figure out a human. They wouldn't have the capacity to figure out any reasons why we'd be a threat, other than "hungry for roach meat." The concepts of spreading infection by defecating
everywhere (or even the idea of "infectious organisms"), or of killing roaches because you enjoy the squishy crunchy sound it makes, just would not occur to them. Maybe aliens would come here for a summer holiday, and it would amuse them to increase the benzene content of the oceans from damn-near-nothing to 37%, due to a sense of humor that's beyond our comprehension. (Seriously though, benzene is hilarious stuff!)
Another way of looking at it is a naive mouse wandering around in an open field, singing loudly while wearing a bright orange vest. By chance circumstance, his family and friends knew nothing of the existence of hawks. But darn it, he's going to find out that what you don't know about
can hurt or eat you.
And here we are, broadcasting our orange vest of EM radiation in all directions.