I find that Bayes Theorem is more useful for decision making or limited classification.
If you have a smaller set of choices, and a training set of examples that select from those choices, you can use it. It answers the question, what is the probability that a given choice among the choices is the correct one.
This is good for sentiment analysis (positive|negative) or Email validity (spam|not spam).
https://class.coursera.org/machlearning-001/lecture/243
http://www.programminglogic.com/bayes-theorem-with-examples/
For topic selection, because of the sparseness of Natural language, and the fact that some words which may show up less frequently may influence topic selection more, I like TF-IDF better.
http://en.wikipedia.org/wiki/Tf–idf
Topic Modeling
http://videolectures.net/icml07_mimno_moht/