OCFA is currently running a trial of using a couple of squad style vehicles as medic units (M31 and M57 I believe). The reason in the past for running the medic units in ambulances was in case a firefighter was injured on the fireground they would use an OCFA ambulance to transport. An analysis revealed this had occurred very few, if any times over the years.
More info at about 6:15 in this video.
http://vimeo.com/28675505
More info at about 6:15 in this video.
http://vimeo.com/28675505