Depts have certain staffing and deployment issues to achieve in suppression. If we're talking about a fire based dept that does EMS transports, we're talking about maybe a 20% RIF if EMS were to be dropped entirely. Just because suppression units have a lower call volume than EMS doesn't validate a reduction in those services. Deployment and staffing issues still need to be maintained regardless. Here's a video I posted some time ago that addresses this issue, from my own dept:
http://www.youtube.com/watch?v=a_K-K6o5cGc
The main financial arguments (not arguing effectiveness, just financial) for fire based EMS are utilizing idle (but still necessary) suppression units, combining the command structure, flexibility in staffing and thus lowered OT, and paying less benefits, paid time off, training, equipment and such than if you had an exclusive third service instead. Divorcing EMS from an EMS transporting FD would result in higher operating costs for fire and EMS collectively. The suppression units would still need to be staffed and deployed, however.
The RIF would be equivalent to the number of EMS positions. To reduce fire staffing further would require a reduction in suppression service.