I sort of agree, but to be fair a nice habitable planet could be a really rare and valuable resource in and of itself, assuming there are any life forms out there with biology similar enough to our own to find Earth desirable.

Also assumes there aren't a bunch of Earth-like worlds that are otherwise uninhabited that would be much easier to colonize than one with potentially hostile/poisonous natives.

or that the potential alien invaders are not adapted to a completely different environment than earth's

Earth is a rich, temperate, naturally life-sustaining planet with a strong protective magnetic field. Regardless of what form of life an alien species would be, that may be of interest in and of itself based on mere rarity. Why bother terraforming if you don't have to?

Besides that, our solar system is otherwise quite rich. Middle-life, medium-sized star has a lot of mileage left. Being a single-star system, the planetary orbits and climates are naturally stable. Mercury has an enormous core of iron. Venus has a tremendous amount of energy stored and is a prime terraforming target for a civilization with the right technology.

We have 4 enormous gas giants, one of which forms a whole planetary system in and of itself, and could conceivably be collapsed into its own star.

The moon Titan is a giant ocean of liquid hydrocarbons.

This doesn't even get into the rich material wealth offered by the asteroid belt, Ceres, and Saturn's rings.

I'm sure there are better systems than ours from this perspective, but we're far from the bottom of the barrel. I'd sure as hell move here.

Another point is this, perhaps the potential of humans themselves presents a threat to another spacefaring civilization. What if they've seen species like ours rise up and conquer, and they'd prefer to nip it in the bud instead of waiting around for us to become more powerful?

We might be tasty.

Not me. All gristle and bone. I taste like your used glorkin.