Planetary defenses can be very powerful when properly used, especially in games against the AI.
The main point to remember is don't build them unless you need them. They are just wasted cash if they never get used. Against the AI, they are great to create chokepoints, since it won't fly past. Against real people, it takes phase disruption to create a chokepoint in a multiplayer game.
Some things to consider for multiplayer ...
Repair bays are extremely powerful at keeping your fleet alive. 2-3 repair bays can totally swing the course of a battle when you are defending a world with your fleet. A couple turrets sprinkled around your repair bays and key structures is also handy.
A few hangars sprinkled around your empire with fighters will chew up scouts trying to cross your system, denying the enemy intelligence.
If you are playing on a big map, it starts to become useful to have some defenses to protect against raids. Yeah, the defenses will die, but they should give your fleet time to arrive. Would you rather lose some defenses or the planet to a small raiding force with siege?
Fleet upgrades start to seriously hamper your economy at some point. If the fighting seems to be happening more at your worlds, defenses are actually very economical for the cost. They are meant to bolster the strength of your fleet.
The thing to keep in mind is defenses don't win you the game...but if you know when to use them, they CAN keep you from losing.
I'm also interested to see what Entrenchment brings to the game as far as strategy. I'm a little apprehensive, since the last half of the game can already be a grind against players who have no hope of winning because they did nothing but load out their few worlds with defenses instead of building a fleet. For people who play epic length games against the AI, I think Entrenchment is gonna be pretty great.