That and, in a wider sense, a species' colonization of other worlds, is a very good point.

We are all made up of carbon, oxygen, nitrogen and hydrogen, with sprinkling of some trace elements. Yet there are combinations of these elements that are deadly toxic to us. Heck, even the same chemical has a different effect on us depending on whether it twists left or right. There was an excellent sci-fi book touching on the interplanetary biochemical compatibility. I thoroughly recommend reading it.

In short, I sincerely doubt the aliens would find us palatable.