The statistics in the CoNLL 2004 and 2005 shared tasks (Shen and Lapata, 2007) shows that the semantic roles of ARG0/agent and ARG1/patient account for 85% of all arguments and most likely act as the centers of the local focus structure in discourse due to the close relationship between subject/object and agent/patient.
it is appropriate to model the centers of an utterance from the semantic perspective via semantic roles.