This is exactly the problem. We have so little to offer any space-faring civilization that could possibly reach us that their motivations for even coming out this far should make us wary. Especially when our particular planet seems to be so rare in the universe... There may be hundreds of Earth-like planets out there that we haven't even discovered, but they are dwarfed by the number of rocky and gassy dustbowls with hostile environments unsuitable for any sort of life we can fathom. For thousands of lightyears in any direction, our planet (as far as we know) is alone in supporting life, and to anyone looking for such a habitat, our verdant paradise might be a beacon in the dark.
I doubt our species would be capable of interbreeding, and if their technological advantage was sufficient, I doubt they would even bother to eradicate us. Consider how much care and sympathy a building contractor has for the ant hills he bulldozers over, never even realising they were there. Naturally we have a little more reach and depth to us than ants, but we might well find ourselves so outclassed that we're treated little different than scavenging bears on their new frontier homestead - no more than pests to be fenced out.
Earth currently has nothing to offer a technologically advanced civilization at the moment, so if were to happen within the next hundred years or so, it likely would be for it's natural resources.
I found http://simplebutgood.net/?p=123 a while back in the newspaper, which may suggest that earthlike planets may not be as rare as previously thought. We are just now within the last few years really starting to find more and more planets in space, most of which would likely be uninhabitable by any form of known intelligent life. Let's not forget that lower, more simple forms of life can survive in some pretty inhospitable locations (even here on Earth) which theoretically could support more complex life in the far future. Once upon a time Earth wasn't exactly a hospitable place to live.
Interbreeding I think would be an unknown. Could be possible, then again it very well may not be.
It could be possible that life we may find out there won't be hostile at all to us, it could be that they are simply explorers, much the same as human beings are natural born explorers. If we weren't explorers, then America never would've been discovered, and we'dve never put a man on the moon, and now have set our sights on traveling to Mars, and possibly beyond.
I can see how some people might be saying that it is too much, too quick, too soon. The unknown tends to scare us, and the thought of an alien race from another planet gives us images of War of the Worlds in our heads. Reality, when/if that day ever comes when we do make first contact, probably won't come to hostility unless we are still a war happy people that still can't get along amongst ourselves, let alone another intelligent species.