The reason you have to tear it is because once it is played, you can discard any three characters in play or set aside that you want that aren't protected and the discard can't be stopped (just insteaded). Now, there are plenty of protect/instead cards now so plot has lost some of it's umph, but when plot was first printed, there wasn't any instead cards (correct me if I'm wrong), and protection from discard was really limited to Altar of Incense or similar cards that weren't blanket protectors.
With the fact that the card can't be stopped once played and back when printed there weren't nearly as many options for mitigating the damage it could do, tearing it was deemed to be an appropriate cost to have to pay; especially considering that opening patriarchs packs was done much more often as they had some of the best cards at the time of their printing. Recently (relatively), Cactus printed of Haman's Gallows to try to make up for the fact that pot has lost a fair amount of its bite with the number of instead/protect cards floating around, but that still costs you a card slot. TechnoEthicist mentioned in home/house games just playing it as a "remove this card from game after..." card, it's what my group does. Saves cards and money, and Plot doesn't pop up much in our games, anyway.
