In my book teaching them to lead with the elbow is the same as teaching them IR. If you just let them go out there and pitch and let nature take it's course most them will be bowlers. However most pitchers that stick with it will end up with some form of IR but it will be minimal. Most pitchers that get to the point where they can throw at 50 mph will have their hand on the side of the ball at 9 o'clock and their hand behind the ball at release (IR).
This is an interesting question, how do elite pitchers develop IR if it's not taught? My theory is that when pitchers start to learn the curve and especially the rise they develop better IR. The crucial spot in the downswing is between 8 and 7 o'clock. Ordinary pitchers are rotating their arm at this point, they are surrendering to the impulse to push the ball, and losing the advantage that IR adds. In order to throw the curve or the rise the pitcher must pass through the 8-7 o'clock position with their elbow bent and the hand still on the side of ball. Even though they may not be putting the right spin on the ball by just trying to throw the curve or the rise pitchers are putting themselves in a better position to fully utilize or execute IR. This is why I am reconsidering my thoughts on the bulletspin fastball.
Practicing screw ball spin helps also....it makes them try to lead with elbow. I tell them to think about leading with the pinky finger when they can't get it.