Problem is that any active jamming system would be powerfull at close range, not at long range. So missiles could still be fired from far away. An active jamming system would probably reduce hitting percentage at the target - however, I think the actual system does work in that way, right? The only "advantage" would be to have separate ships which would allow protection for several ships - not for oneself.
A different solution would be to use "jamming ships" in front of the main battle fleet. There they would have to be targeted first to get missiles through. The MaxRange of such jamming tech should not be too large. But they would be able to reduce effectiveness of super long missiles.