Stretch gauges may be a pain to use on a motor, especially under the car in an emergency bearing check. But rod bolts are nothing more than a piece of steel with a given stretch rate that will repeat as long as it is never exceeded past a maximum value, which if I am not mistaken is about 33 percent more than the specs given. It is my understanding that rod bolts are just like springs, they will stretch, compress, or twist a certain amount for many cycles(sometimes millions) like they have to during use. Just goofing around in the shop one day I discovered I could stretch a rod bolt for my race rods by .0002 just with my finger tips. That to me proved the point that the spring or stretch rate is linear.
The stretch value puts them in tension at about 3/4 of their maximum strength so they have room to survive a long stressful life in the motor, yet still keep the rod caps together enough to not let the bearings distort too much or spin. I just don't see how a torque wrench can be consistently as accurate due to varying friction conditions.
Same goes for a torque plus twist method. It is more accurate than a torque wrench, but less than a stretch gauge IMHO.