It might. Current humans analyze stools to determine much about a person's health.

That's why President Bush uses a port-o-potty when he goes overseas. The secret service doesn't want foreign govts analyzing his stools and discovering he's a boozer (and God knows what else).

That's not to say every group of 'aliens' uses this procedure. While it is convenient to think there is one homogeneous group of visitors to Earth, the truth of the situation is far more complicated.