[CITATION NEEDED]
That's a hell of a claim. That using a Rapier with an offhand (sometimes a weapon, sometimes cloak or even a buckler) was relatively common, absolutely supported by history as we know it, but that more than 50% of the time in "many times and places", people used a dagger with a rapier? I think that's going to be hard to support outside of specific cities and specific faddish time periods. I can think of a few times it might possibly have been true, but many where it was definitely true? I cannot.
However I do agree that it's dumb to not allow it at all or require specific classes or feats to do it.