Hitzy ... good point ... never thought of that, but you are absolutely 100% RIGHT ON!!
[My minds eye pictures a not so happy shooter jumping on his Op rod and saying special words ... ]
[;{)
Rotating the bolt in a bolt action rifle, the spiral camming of the locking liugs exerts tremendous leverage and pressure to extract the case. Pulling straight back on an op rod in an M14, you have very little rotation and lots of hard pulling to do. Nowhere near as efficient.
If a case sticks, you will have to resort to butt on the ground and stomping on the op-rod. There is very little leverage to open the bolt with the op rod.
You can't just turn off the gas system on a stock M305/M1A like you can on an FAL. If you take out the gas piston, you'll still have gases coming into the gas tube. You'd have to put in one of the adjustable gas plugs.
I have tried this some times, only taking away the gas pistong and the plug. The groups is getting about 1/4 inch smaller.