Using tarot cards as a focus for magical efforts is not much different then using any other visual focus. There are a few details that must be considered when using tarot cards that would similarly be a matter of some care when using different divination tools for magical foci. (Runes will be getting their own set of notes and such.) Firstly, the question of how the cards are handled must be dealt with. (Yes, I know, it was a pun and no, I am not repentant.)
If one wishes to keep the cards in usable condition, this limits how the cards can be manipulated. They must be carefully kept clean, dry, and unbent. Obviously, they must remain whole. Depending on how one is performing their ritual, the cards may be put into a protective enclosure (ie. Plastic sandwich bag, small laminated pouch, etc.) so that they might be exposed to spell components. It may also be an option to place the card where they stand/lie in a central location of the ritual space where the energy of the spell work is focused and the ritual gestures and actions are performed about the card. In either case, the cards must be physically unchanged from the beginning to the end of the ritual. Once the ritual ends, the cards must then be 'grounded' (cleansed of the residual energy of the ritual and restored to a magically neutral state).
In rituals where the destruction of the card is part of the process, the limitations on how the cards are manipulated are only imposed by that of the spell work itself. Sometimes, however, one may wish to do a ritual that would require the destruction of the card but they do not have cards to destroy. In these cases, it is good to use a bit of tracing paper and copy the key visual elements of the card and what ever notation is necessary (ie. Justice's scales and sword and the word justice along with it's numeral). This copy can be as detailed as is necessary. Once it is made to a satisfactory degree, it can be used exactly as the original card would have been.
Because the cards themselves are loaded with meanings and the associated spiritual energy that comes from these thought-forms, it is easiest to use the card that best matches the desired outcome of the spell or the specific target of it. In these instances, the court cards of the Minor Arcana are used as representatives of people more frequently then as representatives of concepts. While it is possible to use the court cards to represent concepts, it will require a bit more focus because the thought-forms build up around them are swayed more strongly towards people. Sometimes, the thought-forms are easily disregarded, such as in the case of the pip cards.
In either case, approaching the use of a tarot card (or multiple cards) as a magical focus can work as a type of 'shorthand' for the goal of the spell work. There are several different spells available on the internet that incorporate tarot cards and at least one of this variety in the writings of Raymond Buckland (I think it is in his Complete Book of Witchcraft). The use of tarot cards as a tool in spell work is not as extensive as something such as candles, but it is a practice that seems to have been on the rise since the 1980s. I think this is due to a combination of factors.
The rise of accepted witchcraft media and the boom of practitioners mixed with the cost of tarot decks dropping as the number of varieties increased is easily the largest reason this has come into play. I would also argue that this has grown to a more common form of spell casting is because the thought-forms used are in many respects 'pre-programmed' and easier to attach to one's spell work rather then the effort of creating one from scratch. I personally don't use tarot cards for magical focuses but I have known people who have done so with great success. Thus, I believe it is a legitimate practice that can prove highly effective when done properly.