I watched a playthrough that was posted here the other day, and the rebel player used two flotillas, each with Jamming Fields. His logic is he wanted to keep his fighter screen alive, which consisted of X-Wings and A-Wings.
I noticed during the game that the JF's ended up hurting him a lot more than helping him, as his A-Wings each lost a counter die and his X-Wings didn't have the punch to finish off TIE fighters.
The only consideration I can think of is perhaps shielding bombers in on an attack run.
Am I missing something here? What do you guys use them for?