I'd argue it has as much of an objective as any other shooter though - just on a much larger scale. Control and hold the map/planet. If you push them back into their safety zone, they'll either push back and try to take back what's theirs or (more likely) go for an easier target on another planet. And then that means you go to another planet. It's just like a map change, but without the "Victory!" screen.

I realize I'm biased, but if you played PS1 I think you would understand what I'm trying to say.