... or might be poison.

Better make some friends with Ninja Aliens.

Or we could just sick our illegal aliens on them. They'll wake up to have their jobs taken and ray guns jacked.

I am transferring current human tendencies on to an alien species but humans do dangerous and stupid stuff all the time for no practical reason. We don't "need" to do quite a lot of the things that NASA does. The countries that are talking about going back to the moon certainly don't "need" to do what NASA already did 40 years ago. The assumption that aliens would solely base their actions on "need", especially when humans do not posses the same inclination, is at least a little presumptuous.

Any alien ship that gets close enough to Earth to start a Twitter account will move on and leave us alone in short order...

Any alien race that came to Earth would likely observe us first before attempting contact, and the first thing that would strike them would be man's inhumanity to man. Leaving bleeding people on the street, the overwhelming violence in our media, and the history of war that goes back farther than the history of writing information down. Literally, killing each other was important to us before any method of transferring knowledge that didn't involve talking was.

This will result in one of two outcomes. Either said hypothetical extraterrestrial race will steer clear of us, probably slapping an intergalactic biohazard label on our solar system (could this be why we haven't met any aliens yet?) or the other alternative, they preemptively wipe us out utterly to protect themselves. While I think either is likely, unless you use a method of killing us that affects us and only us biologically, it would be hard to beat humanity in a conflict. The one thing we do better than anything else is kill things. The fastest developmental curve for any technology in any human culture is that of weaponry, and almost every scientific advance we've ever made has been turned into some method of killing. I don't have an opinion about it either way, but in any kind of conflict other than a brutal, effective and overwhelming surprise decimation I think humanity will eventually have an edge.

What if they are more sadistic than us? Hey, look a new breed of alien pets. I want one daddy!

I don't think that they would necessarily do EVERYTHING based on need, but life in space would tend to create a certain desire to be economical in action. You wouldn't waste resources, nor would you take actions that wouldn't yield SOME concrete benefit. And if the concrete benefit of hunting humans with a napalm projector is better morale, then maybe they'd do it. I tend to think not though.

Gort. Klaatu barada nikto.

What if we are unable to help save someone else's home world in the future and they can back to our past and destroy us?

Then they're vindictive little bitches, because at least we tried.