I agree with ThoraStorm on that the future, in theory, is constantly changing or at least the minor portions of the future that don't necessarily effect the ending result (which is highly subjective).
Too many readings (regardless of the method) can scramble your interpretation of the future and confuse the energies put into your readings. Make sure to cleanse your deck after every different subject or every three-four readings.
Tarot cards are as informing as you make them. They can also be used to guess the past to an extent (whatever part of the past relates to the part of the future in question).