I separated my LH shoulder about 2 years ago. This has left some muscle imbalance within the shoulder, and the wrong muslces being used to move it (according to physio). What I do know is my R shoulder has some muscle mass under my collarbone at the front, and my L has a hole.
As recent as February I was out of the pool for almost 6 weeks due to a shoulder flare, caused by hard intervals in the pool. Undoubtedly down to poor technique, and the wrong muscles pulling the shoulder. I struggled to lift my arm above my shoulder at one point.
I'm sure paddles have their uses, but until my shoulder is strong enough they're a no-no for me.
Normally I'd agree with the specificity in training. Even so far as to agree with Barlos; I'm pushing through the stroke more than I used to, and trying to glide more which is really helping. Single arm drills are fun. I don't make much progress left-handed!