So think about ambulance response times in your area. The longer the response time, the longer the round trip, the more advisable it becomes -- especially for injuries that EMTs can't help much -- to put the victim in a car and floor it. In rural areas the response time often exceeds half an hour, sometimes even more than an hour. But if you live in a area where the response time is very fast, having expert care may still be preferable.
EDIT 3/9/18: Also, if you live in or otherwise frequent any area where ambulance times exceed reasonable safety limits -- about 8 minutes -- then consider learning first aid for serious issues such as CPR and bleeding control.