by Hannibal Rex
Any additional drive is one less computer/weapon/hull. That's a big deal for cruisers and interceptors, and you can only have two jump drive dreadnoughts.You can usually wait to build your defenses until you're attacked, especially if his ships are sub-par in combat.
It certainly forces you to keep materials and actions (i.e. money) in reserve, but if you're preemptively building defenses in every single border system just because someone has the jump drive, you're doing it wrong.