Sharing a pay-per-ride Metrocard with your group

In this thread, it is said that a Pay-Per-Ride Metrocard can only be used at one station four times within 18 minutes.

I know the unlimited Metrocards 'lock out' after a single swipe which makes perfect sense -- so more than one person cannot 'share' an unlimited card with others in their group.

However, if the 4-swipe lockout for PPR Metrocards is indeed in effect, what is the rationale for this? Why not allow a group of 5 or more to share one card? My girlfriend and I share one card, whoever goes through the turnstiles first hands the card back to the other.

I just guess that the four-person limit is to somehow rule out scams. In our city we have a lot of smart guys figuring ways to make cash, so maybe the MTA decided that most groups of friends would be four or fewer. Note also the way the system uses up all four transfers in one swipe, not four. Maybe it is a question of keeping the card's memory of transfers limited. Note that the 18-minute rule is calculated because the Metrocard system uses tenths of an hour rather than cutting an hour into sixty pieces. Eighteen minutes is 0.3 hour. Much simpler time categories.