We know that one of the main priorities of the SGC is to explore other worlds and interact with the inhabitants of those worlds, hopefully to form alliances and trade technology. Now, there are probably countless other worlds that send out teams to explore just like the SGC. We know that the SGC sends in the MALP before going through the gate to make sure the cost is clear, and determine whether or not the gate is accessible (iris or something).
What I'm getting at is: What if these other explorers do not use something like a MALP, and they decide to make contact with Earth, only to meet the fate of the Iris. I know that the Iris is there to protect Earth from enemies and the such, but how many times do you think that has happened, where the thud turns out to be friendly travelers. Also, if that ever were to happen, I'm sure that the world would send 2nd and third teams to find out what happened to the 1st, and when neither of those came back, they would suspect Earth as being an enemy.