I often include in my decks a single m**k*d or sp**t*d card so as to be able to do a Jonah card poker deal trick.

What others tricks can be done with just a single d*t card?

It can be helpful for equivoque of several face down cards because you can keep track of where the target card is. (let's say you have 5 cards in a row - you can let the spectator mix them up and have an easy time keeping track of whether they select it or not)

It is also useful for several hands off type opportunity forces like the table spread force or the toss spread force, so you can be certain they took the right card.

Although I don't use a P****L D*T, I've been working on a number of effects using these types of cards.

A great effect is Intuition with Cards in Royal Road to Card Magic. Any effect that uses the key card principle can be done with the deck face down. Even better using it as a distant key or floating key. You can also use one to do the classic force without having to cut the deck. You can cull the aces to the top of the deck without having to look at the faces of the cards. Scarne's Calling the Cards is a lot easier using one of these. If you are using a stack, it provides a reference point without glimpsing or sighting a card.