http://www.collegedata.com/cs/data/college/college_pg02_tmpl.jhtml?schoolId=1138 indicates that USC does not consider “level of applicant’s interest”, so ED may not be all as helpful there as at a school where “level of applicant’s interest” is considered.
Applying ED can still be useful – if the result is admission, then the student is done and need not send any more later deadline applications (but should have them ready to go in case the ED result is rejection or deferral to RD). However, the student gives up the opportunity to compare financial aid offers with other schools, and the school may be less prone to offering merit scholarships if the student has already indicated a commitment to attend.