According to the American Heart Association's statistics, the outcome of CPR is mainly a question of response times and technique (proficiency of the crews involved).
Cardiac arrest is an extreme emergency (basically the patient is already DEAD !) - it wouldn't be fair to evaluate the usefulness of EMS based only on the outcome of cardiac arrest cases...
During my 15 years working as a medic, we managed to resuscitate a dozen patients (who made it home with a favorable neurological outcome ) - but I guess that we must have saved many patients that were having extreme emergencies and would have probably died without our intervention (complete respiratory arrest for asthma patients, severe pulmonary edema, cardiac arrhythmia, etc.)
Better pay, better working conditions, better education would contribute to a higher motivation amongst EMS workers - the end result would be a better service to the community.