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Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a creature you control may become lethal if The Walls of Ba Sing Se leave the battlefield during that turn

At least everyone is taking four damage, but they could be taking eight or even 12 damage depending on the number of snitches

However, if you cant immediately follow it up with tokens then its probably best not to play it as it will just get removed

The creature gains haste

If you ran enough of them, it may even be able to change the tenor of the deck slightly, like running more Pact of Negation or other choices