The reverse punch as you described in the kata is a different technique than the one used in sparring. I think the separation on usage has a lot to do with Seiei Kan's focus on tournament kumite.
A point is given for contact with the puch regardless of rotated or not. Therefore, economy of motion becomes key.
However, in a real situation one must be cognizant of the desired target and effect of the punch.
For example, a solar plexus strike would benefit more from a rotating punch. This would bring the knuckle deeper into the solar plexus, leaving less chance to catch on the xyphoid process (the small bone projection at the bottom of the sternum). Also, because the rotating punch is designed to be thrown at solar plexus level (on the mid-punch) it will naturally strike at a slight downward angle, thus creating the desired effect on a solar plexus strike.
Conversely, we have the "blitz-punch". This is just my term for it because I throw this punch a lot when I blitz the opponent. It is short, effective, and harder to detect or divert. That alone creates certain advantages for it.
A rotating punch, because of the rotation, is easier to bypass than a rotating punch. This is because the elbow ends up on the outside of the arm, which is easier to divert than the bottom elbow of the blitz punch.
The blitz punch is quick to the target which makes it good for the surprise strike as well as a lead-in strike. Since I use it to blitz, I strike with it but am prepared to fire off the next technique. The rotating punch works well when the opponent is coming to you instead because of the extra penetration.
The blitz punch works well on vertical targets such as the brachial nerve centers underneath the arm. For this reason I also sometimes sweep an arm not only away from me (enough to miss me anyway) but also at an upward angle. This opens up the nerve center for the quick strike as well as removing most potential counter-movements of that opponent's arm except an elbow strike.
Both are useful strikes, just different. In kata I consisitently see the rotating punch. I believe this is most likely because it is the more traditional strike and so was incorporated into the kata. In kumite, I say it's whatever gets the job done.