yes and no valefore, since the respawn is so random, you may never have anything TO defend, lol. Then also, your ships won't stand much chance against some players, like ronald or ricu.
About ships not standing up to attacks from other players...
Just an odd thought, but shouldn't truces still be recognized, at the very least on the alliance level. I mean I had Ronald come through one of my systems and clean out my ships there, although for his rank it took quite a few attacks maybe he was using only one fleet. Yet his alliance and mine have a truce between them.
All in all, it is a good system.
This had happened while I was around rank 150 with ronald at rank 1. It took 5 attacks to clean the system of my ships. As I had said then, it is possible that he was only using one fleet, but I don't think it to be likely, as I don't know many people who attack with only one fleet. So, another theory is that ships guarding a star recieve a bonus to hitpoints and defense. But I've noticed that ships around a star tend to stand up to attacks better then normal.