In the case of a cardiac emergency a BLS ambulance can do a lot, as can CPR-trained bystanders. Early high performance CPR and early defibrillation (assuming a shockable rhythm) are crucial. A patient without circulation can be dead in ten minutes from the onset of the emergency so getting them to the hospital is not the priority. CPR and defibrilation is, no matter where that occurs. It is really difficult to do good compressions in a moving ambulance unless it's equipped with the mobile automatic machine which is unlikely in a BLS truck.