Using a rapier without employing your offhand was extremely rare, which was, I believe, the point he was making.
That offhand might be employed holding a dagger, a buckler, a cape, your barehand, a glove with a mailed palm, or, in rare cases, another rapier. The most common of them, for various reasons mostly involving the best way to deal with a light thrusting sword, was, in fact, a parrying dagger, which was a dagger whose primary purpose was parrying.
Not enabling that as a matter of course is indeed a bizarre choice. But of course, it's always bothered me that Two-Weapon Fighting in D&D isn't primarily focused on improved defense.