In the urban environment, seperate services works the best. The tax base allows for fully staffed seperate services to exist, as compared to more rural regions. As you get progressively more rural, a combined fire/EMS service functions more effectively, since staffing/deployment becomes more challenging as the population becomes less dense, and the tax base drops.
As you get progressively more rural, the fire suppression units typically see less of a net utilization, and EMS will be short staffed/deployed and therefore stretched thin due to budget constraints. EMS txp times upwards of 1/2 hour, plus return time can put a unit OOS for well over an hour or more and also out of their first due. So, since fire and EMS have a somewhat similar staffing and deployment arrangement (very different from the police), it's the best use of resources to have the FD rtrain for and run EMS first response, and possibly even have dual role providers do EMS transport as well, which saves on OT, hiring, and mandatory holdover, since personnel can do both jobs.
As far as job proficiency, FF 1/2 and a Paramedic program is less education than an Associate's degree. My FF 1/2 was 22 weeks FT, and my medic program (from an accredited institution, NY Methodist) was 13 months, which awarded me one year's worth of credits towards the EMS AAS. As such, the argument that dual role FF/EMS employees cannot possibly do both jobs well is weak IMO, provided the personnel rotate from suppression to txp units, and in-station drills and multi-company drills are regularly practiced, as is the case in any quality fire/EMS organization. Basic firefighting is not mentally challenging, and neither is EMS (I've got ten years in, 7 as a medic), so doing both is certainly less difficult than studying and training for a profession that requires a Bachelor's degree or greater.
Also, many more FD/EMS departments pay well, have a real career ladder, and also offer excellent benefits/retirement than does the hospitals, third service, and certainly the privates. From my perspective, aside from a select few third service EMS and maybe a hospital or two, fire based EMS is the only EMS delivery model that's sustainable for the long term for the employee, and also offers a secure financial position.
Look at the combined LEO/fire/EMS organizations. The ones I've seen belong to small jurisdictions with a presumably inadequate tax base to fund seperate services.
The funny thing is, LEO's have probably the most stress in Emergency Services, followed by EMS, with fire having considerably less stress. I wouldn't want to be a dual role LEO/EMS employee, since both positions are high stress and the burnout in EMS will only exacerbate things. At least with fire/EMS you should get a break from riding the box and have more down time.
My $0.02
