Conceptually, the third seems a different way of doing it for no apparent reason.
The only reason is the evolution of the Inform library over time. The noun and second noun variables are early features (whose history stretches back to Inform 6 and 5). The current action is a recent feature that encompasses them in a more structured way.
It is possible that a future release of I7 will deprecate noun/second, or recast them to be accessors into the current action. However, there's no urgency on that -- it's just, as you describe, a small conceptual cleanup.