Admittedly, anything can be gamed...infact that is why munchkins are munckins they find ways to game the un-game-able.
Your defences are per system, so defences in Sol are based on what you have on Earth mainly. The NC deploys smaller bases for defences of colonies, but as they found out that only works if the colony in question has minimal atmosphere since they are laser bases. So the development of a short ranged meson weapon that can be used instead is prioritized.
But still, any sizable colony would probably like to have defences. Now if you say "the game only requires me to defend one place per system" then fine, against picketing settlers you are safe. Against marauding alien scum you are easy pickings. My view is that taking a step back from the spread sheet and saying "if this was real, what would I do" often helps avoid these sort of essentially fundamentally absurd situations. The cost in terms of time, and resources to establish any sort of extra solar presense makes defending it a requirement at some stage. Failure to do so will have more than "role playing" consiquences regardless of how much you dodge the bullet in terms of game mechanics.
Aurora allows you to many things, a lot of them rather less than wise...playing silly buggers with your defenses heads my "less wise" list.
