Defenders serve several purposes, but not the ones you might think.
The main purpose of a defending ship is to tell rival civs that the planet it is guarding is not the easiest target. In this case, any ship is better than no ship and Evil Stormbringer's empty hulls work quite well.
The second purpose is that any defender blocks a transport from being able to invade a world. Again, even an empty hull works quite well.

The last purpose, and the most obvious, is to fight off enemy ships. My experience, and that of others from what I read on the forums, is that this almost never happens. In my experience 99 out of 100 defending ships NEVER fire a shot! This has lead many to abandon the idea of the defending ship entirely.

I have come to a different conclusion and now use a different strategy. I begin building each of my worlds one defender as soon as they become available. As I gain military technology, I design better defenders and have these built. I used to then upgrade all the older defender designs to the newest design, but I found this extreemly expensive and unproductive. When I get larger hull classes, I build defensive ship on this hull and have ALL of my worlds build a new defender. As each larger defender is built, I convert it to and offensive ship and send it to a rally point. This allows you to convert your fleet without the massive expense of converting them all at once, while doing a complete conversion.

If 99% of them are never used, what is the point of all of this? The AIs in GC2 are written to look for weaknesses and exploit them. If you have no defending ships, they will send a wave of unescorted transports toward you worlds in the hopes that any ships that get through will give you problems. If all of your worlds have one defender, the AI will send a fleet to pick off your defenders one by one, and then sends the transports.
The purpose of this method is: Every world builds its own defender, which allows your stronger worlds to concentrait on other more important things. With each world having a defender, the AIs don't see easy prey and so concentrait their forces, which are more easily countered. Larger hulled defenders can usually due a good amount of damage to even a large fleet. After two or three encounters, they will stop attacking as they have been weakened to the point that they no longer see any battles that they can win or they are destroyed by a defender. This blunts their offense until you can get your own fleets into position.

How to build a defender:
The 'Defender' design is actually not a very good defense ship, as it is actually a heavy fighter. An optimised defense ship contains the following:
NO engines or life support! These are wasted if the ship never leaves orbit.
One of each of you best defensive systems.(Note: early in the game this is not possible due to space limitations) This can be further refined based on your enemies weapon choices. If no one has beam weapons, shields are dead weight.
Put as many weapons in the remaining space as possible, balancing them between the weapon types as best as possible.
This type of ship has a distinct advantage over enemy ships due to the fact that they need engines and life support just to reach you. You will usually have a 30% advantage in firepower and defenses compared same tech and hull size ship. By the time I get to large hulled defender, they tend to devistate enemy fleets by themselves.

Scincerely,
Scintor