Even though (statistical) phrases are more representative units for classes than single words (Caropreso, Matwin, and Sebastiani 2001), they are so sparsely distributed that they have limited impact during the classification process.
it is not surprising that the best scoring multi-class, multi-label 3 classification results for the well-known Reuters-21578 data set have been obtained using a BOW representation (Bekkerman and Allan 2003).