You keep calling it a rule, but from the link you provided...
Finally, it’s not an official rule, but we recommend setting aside the hands you're mulliganning away until you get a keeper. That saves shuffling time, and we're all for minimizing shuffling 100-card decks.
There is no mention of it in the official rules because it isn't an official rule.
This kind of reminds me of one LGS I went to where they allowed you to start off with three basic lands in your command zone with the ability to play one from there instead of out of your hand except on the first turn. To me, this resulted in bad deck building as they were putting less than the "proper" number of lands in their deck as it allowed you to keep a one land hand. The way it effected their deck building showed when they played in playgroups outside the shop as they were consistently mana-screwed.
It may or may not have effected the deck building of your group, but I'd wager to some degree it has.
There is no mention of it in the official rules because it isn't an official rule.
Building: Varina
It may or may not have effected the deck building of your group, but I'd wager to some degree it has.
Building: Varina