I love the new stellar defense modules (and the equivalents in various races). Finally, ships designed for defense are forced to defend! The implications for newly conquered space (or influence bases) are also pretty fun.

However, I think that you should be able to stack multiple stellar defense modules on your ships. After all, these aren't fleet modules, and so you aren't going to get runaway stacking of the bonuses. What it would allow you to do is make truly powerful defense ships by axing more and more of their ability to operate outside your influence. The additional firepower they would gain would be mitigated by their lack of flexibility.