If you look at older logs this point was argued to DEATH!

PJIs were always never quite right.
First they blocked all traffic, outgoing and ingoing.
a. not realistic in Multiplayer
b. Stalled the game if someone took a choke point
c. Uber Fleet of death only way to combat it
Second they locked traffic to systems you had control too
a. noncolonizable planetoids were open (an enemy could jump from your world to a magnetic cloud to your world)
b. With cannons there would be complaining that you couldn't reach it in time
Thirdly they now delay jumps
a. by too little
b. Not effective for defense due to fluxuating entry points (negates all defense)
c. If it is a delay it should slow enemy movement TO the planet and speed your movement from it.
As for Defense platforms, orginally thier range was up to the phase lane exit. If you clustered your defenses ANY fleet could be destroyed. All it took was a focused field of fire.
After the range got moved back it still didn't take that much to destroy a fleet. Now without any flak defense the cannons just get torn up by strike craft. But if a ship gets in the way a field of fire can kill it.
Hangers and shields are the way to go.