Yes, because if we make it out there, I don't want to deal with having to get rid of the life there just so we can use the space. I am sure humanity will advance to use at least 50% of the space before our sun goes nova, if Moore's law is any indicator of technological advancement.

> so how does it hurt us if some other species do

Liu Cixin's The Dark Forest has an interesting take on this. Basically it's conceivable that the game-theoretically optimal policy is to kill any other sentient species you come across, even if it means you lose access to whatever resources they currently have. This argument falls apart at some point, because it's (presumably) not optimal for all humans to try to kill each other all the time, but it's an interesting idea nonetheless.

I found Blindsight/Echopraxia (released together as Firefall) a very interesting story on many levels, and it covers this kind of subject.