I should point out that I've only played single player with the latest Build.
And that's why you don't understand why they ARE useful. Until you play multiplayer against a player who sneaks a small force past your heavy defenses into the heart of your empire and starts destroying labs/shipyards with impunity while your fleets are fighting elsewhere, you won't understand.
I've used inhibitors to make people curse.
That said, giving them a secondary effect of making phase jumping out of the system to non-friendly space take longer is not really a bad idea. It could still create a death trap if it takes too long, and that's the main reason the devs don't want to make it stop all jumping, but a noticeable but not overboard increase in jump powerup shouldn't be too bad.