The second limitation, related to the ineffective weighting of terms via the the tf -idf scheme, seems to be addressed by the fact that the MaxEnt model assigns a weight to each feature (in our case, POS bigrams) on a per-class basis (in our case, score group), by taking every instance into consideration.
a MaxEnt model has an advantage over the model described in 4.1 in that it uses four different weight schemes (one per score level) and each scheme is optimized for each score level.