Tigger,
If we look at this with the narrow focus of promotion, I see your point and agree. Looking at the big picture (in a very broad view), having all people trained on a truck that are not just FF, but EMS responders as well is a force multiplier. Of course my thinking is affected by my military experience where everyone has a specialty but "every Marine a rifleman, every soldier a trigger-puller, etc."
For what it's worth, I enjoyed being solely EMS when I was running as a medic. We were separate from the FFs. After I left, they merged. I don't know if they make FF get medic-qualified or not (Norfolk, VA).
Enjoy our conversations!
Al