I frequently see AI civs build starbases near their star systems, although it is usually only if there is also an asteroid resource nearby or a resource on a planet in the system. They frequently end up with an avenue of approach on the opposite side of the star system so they can only actually do their ranged attack when you have already taken the star system (since it is itself in range). That window is somewhat closed when the AI gets the Quantum Power tech since starbases get a free promotion giving +1 range, but if you also have it your ships all get +1 speed and can move in and attack from outside the range. It would also be reduced if the AI upgraded starbases to the Omega level, but I have never once seen an AI upgrade a starbase (I have no idea why not, other than that it is relatively expensive for them - the AI upgrade discount apparently does not apply to this).
In the algorithm to pick a location to build a starbase, a star system owned by the AI increases the value (by quite a bit, actually - it may be worth slightly more than 1 resource if the location of the resource is very far from the capital). So do wormholes, only relevant on maps with them, and there is a small bonus for building on top of asteroids (worth 1/14th as much as a having one of their star systems inside the 5x5 area; 50 vs. 700). These are in addition to value added for resources. Currently, the resources found on planets count just as much as those in asteroids which makes the AI even more likely to build starbases that cover those star systems - it sometimes even builds starbases to include unsettled star systems with a planetary resource within their culture area which prevents anyone else from settling there.
The highest valued locations are those that provide multiple resources. Star systems don't start to get coverage until all the better multiple resource locations have been used. A star system with a resource that is owned by that AI is generally worth more than a location with 2 resources that does not include a star system, the exception would be if the 2 resource location is a lot closer to the capital, but worth less than a location with 3 resources. A star system without a resource and no other resource close enough to be able to position a starbase between to get both is likely to be similar in value to a fairly distant single resource location. This is why a system without a resource in it or close to it, or a wormhole, will generally not get a starbase until every resource has been claimed, which might never happen.
So it is far from perfect for defensive purposes, pretty bad in fact, but it does happen some. The later in the game it gets, the more likely it is that you will see starbases with more defensive positions.
Bumping up the value of a star system from 700 to 800, or so, would get more defensive stations built, but it would cause them try to claim fewer resources.
Another possible modification that would help would be to reduce the value of resources that it already has. Currently the 3rd oxygen gives just as much value to the locations around it as the 1st one did. This would have a smaller effect then it sounds like because it can take a long time for the AI to get the resources connected (they sometimes never manage to get some of them connected) so until it actually connects a resource of any specific type, the reduction for resources of that type wouldn't happen and if it can't keep its capital connected to its resources it wouldn't change anything then either. I am very tempted to make this adjustment.
