Taking into consideration only what we know presently, actually traveling to another planetary system is hugely expensive in terms of energy for organic life. You have to build generation ships.... solve any number of space travel problems such as cosmic radiation, the effects of zero-g on several generations of your travelers, renewable supplies such as food and water. The energy requirements to conventionally achieve just a fraction of the speed of light are astronomical. Therefore it makes no sense to spend these resources for a friendly trip when a probe can accomplish all your needs aside from the obvious which is to seed a planet. That task does require transporting organics from one part of the galaxy to the other, and if you do so it will likely be a one way trip. Also the unknowing indigenous lifeforms of your destination planet (us) are likely to not be happy about you showing up and squatting on the planet; so if you've made all the other efforts to get there, you might as well remove the indigenous population as well.
Now, if there are technologies that are as of yet undiscovered that allow for quick and relatively easy interstellar travel, then I will agree with you that friendly visits are an option. I'm going to err on the side of caution though and go ahead and buy a helmet.
A) Colossal space dicks
or
B) Planning on claiming the planet after the genocide is complete.
Either way we can assume that if aliens have shown up, they are here for the only quality worth traveling for: a planet that supports organic life. (Anything else resources-wise can be gathered from innumerable dead worlds and asteroids) If a probe shows up, it could be friendly, or it could just be there to do the dirty work for the parent species.