On the contrary, with a natural jump node it seems that there is no real control over entry and exit vectors, and things like mlojner cannons are expensive and weak. If you've already got a structure built around the node (that controls the vector of entry and exit as well) there is very little reason not to arm it. The turrets don't have to draw off of the station's power source (though I think they should, it'd make sense to have an extra reactor in there anyway in case of primary failure) but they would be more fixed than node defenses. I think such a station should have a connecting actual installation connected to it, with fighter facilities and all as well, but the gate itself might as well be armed.
The other thing is, how do nodes interact with the gravitational forces within a solar system? Do they always exist at a fixed point, while planets orbit nearer and farther away? Or do they orbit like a planet would? If they are fixed, then then permanently stationing defensive guns around a node would be impossible, as they would eventually fall in towards the star. If they orbit, then sentries could orbit with them, but I would think that it would be very difficult to maintain their exact location around and alignment with the jump node. If you have an artificial structure around the node already, then mounting guns on the structure controlling the node keeps those guns pointing where they should, and it keeps them pointing at exit vectors.
