The volume of suppression calls has decreased substantially in recent years. However, there still needs to be adequate staffing for suppression units - for response times, expedition of fireground tasks, and RIT duties should a FF call a mayday. Remember, a typical EMS incident is for one or two pts. A structure fire can involve many victims at once.
So, this can result in fire companies having significant downtime, even with driils, other training, public service/public ed, and station duties.
A municipality may decide to assume EMS responsibilities for several reasons - to utilize otherwise idle personnel, to boost call volume for staffing issues, to collect revenue from EMS billing, to provide adequate coverage to the area not being provided by the previous provider.
Many would have you believe that striving for proficiency in both EMS and suppresion is spreading onself too thin, and therefore unrealistic. It is not, based on my own personal experience and observations especially if the recruit has prior experience in either discipline before crosstraining in the other. Neither discipline is rocket science, believe me, although some may not be cut out for one or the other. Both disciplines require real life field experiences to build on classroom/practical training to be solid anyway. Dual role personnel, who are crosstrained in both EMS and suppression are versatile, and can by used in either role, which helps with staffing, to reduce OT and holdovers. Medics on apparatus can initiate timely ALS care when a medic unit has an extended ETA. The engine medic can also assist the EMS unit, and ride to the hospital if needed.
Whether it be single role or dual role, fire based providers, at least in larger depts, enjoy superior working conditions, benefits, pension, 457 deferred comp, higher pay, DROP, prestige, relative lack of burnout compared to private or hospital based third service providers (typically, as ther are exceptions). Collective bargaining, good PR, and strong political support will do that.
Our dept runs EMS like a third service, except that the personnel are crosstrained and can jump from EMS to suppression as staffing requires.
Having worked private IFT, hospital based 911/IFT, third service 911 prior to firemedic, I could never go back long term. Many from similar backgrounds here have said the same. Life is sweet on this side. I can't see doing 25+ years under the working conditions/lower pay/inferior retirement(defined contribution! I'll have to work until I'm 90!) that non fire based EMS provides.