The point of the hold is it increases you capacity to recruit the muscles, ensure good form, and to not strain the shoulder joint.
Slow contractions only build up a large amount of fatigue. The back responds well to fatigue training so eccentric less training is a great way to increase the volume while doing little to decrease your capacity to recover.
If you are staggering sets of back work in between sets of upper body pressing, the back is already receiving a much larger volume. The eccentric less training increases volume even more. It also increases blood-flow to the muscles, increasing nutrient delivery and enhancing recovery.
Last note, slow contractions will require you to use considerably less load, which only decreases volume.