With regards to Rokkyo, it is part of our curriculum(USAF-WR) and although there are quite a few variations they all have one thing in common.....they affect uke's center through pressure against the normal direction of motion of the elbow joint.
As far as Yonkyo is concerned, not practicing it because it doesn't always work seems like broken logic. If that was the guideline for which techniques were practiced in my dojo, we'd just sit around and look foolishly at each other.
Seriously, as others have pointed out, we have a much better chance at making Yonkyo "work" if we understand that the goal here (as with just about every other aikido technique) is to control uke via their center and not just cause pain. Sure, pain happens sometimes but it is incidental if the technique is done properly.
In aiki spirit,
P.S. Hi Jun and Chuck!