intfiction.org

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.