No, playing cards are not critical to your survival (at least not in the survival situations for which we are designing survival kits). And they should never take space or funds from anything which can help you stay alive. That said, it is worth considering having a deck.
Survival situations tend to have periods of extreme stress and periods of extreme boredom. Having a deck of cards can help with the boredom time, either through games, if others are present, or solitaire. Obviously, you want a deck which are “plastic” or coated , so if they get dunked, they won’t be destroyed.
Many decks come with an ‘info’ card about the cards, which are not needed by any known game. It might be interesting to try burning this, to see how effective the cards would be as a backup fire fuel supply.
An interesting alternative is “survival” cards, which are printed with survival information in addition to the suit and value. The selection of these are much more limited, but are worth looking at. More information is better than less information, and even if the information seems to be of limited value, reading the cards is another way of passing the time, and you never know when you might run across a bit of information which is appropriate to your situation.
Here are the ones I carry: http://www.amazon.com/Sea-to-Sky-A124-Wilderness/dp/B001EVKQKS
Here is an alternative I’ve not tried out: http://www.amazon.com/The-Dont-Die-There-Deck/dp/1594850712
If the print is small enough that it might even possibly be a problem for you, slip a Fresnel lens into the package.