What's the big deal? There's equipment, a para and some techs on scene. 9 minutes, 17 minutes, it's really unlikely that there's going to be any real difference to outcomes here.
And I know American buildings tend to be different to European, giving you different fire conditions, but do you really have casualties who need a paramedic at every structure fire?
I think the reason that so many systems in North America are sending EMS resources to fires is that many dispatch centers are run by FD's. I'm sure the NFPA has regulations for EMS responses.
My old system used to send us to pretty much everything. We spent a lot of time parked outside apartment complexes while they went in and removed pots of burned soup from the stove. It was total overkill. I remember spending 8 hours one day, as a paramedic ambulance in a busy system, watching a fire crew steam frozen hose off a parking lot ("just in case someone gets hurt"). I could have fulfilled this role just as easily from the station 3 minutes away, but then they wouldn't have a dedicated resource, and would have to roll their dice with the system status at any given moment. It was interesting listening to the radio and hearing trucks get dispatched across the city to cardiac arrests, while we were watching what should be a fairly routine procedure.
Rehab at a working fire usually consists of sitting in a bus (a literal "bus" -- not NYC ambulance slang!) with a bunch of FF, running a set of vitals each time they rotate out / go through an oxygen tank. On an ideal day, with a cooperative Fire DC, you identify people who are fatigued / dehydrated / dyspneic, and give them a longer rest period, food, rehydration, and determine whether they need ER transport, and you send the rest back into the fire once they've had a quick rest break. On a bad day, it's a free-for-all with people walking into the truck grabbing a gatorade, sitting outside having a smoke, and then going back in without any real control. It depends on how well EMS and FD work together, and the given staff in charge. Broadly speaking, a lot of the firefighters don't want to admit to being tired / injured, for fear of appearing weak. A lot of them enjoy the rarity of a working fire, and don't want to be pulled out.
I'm sure there's something published about this, but I'm not feeling energetic enough to go looking for it. My personal opinion is that sending ambulance's to anything where it's (i) not confirmed on fire, and (ii) there's no confirmed patients, is pretty pointless if the response time is short. I think there is a valuable safety role in having a rehab sector at a working fire, but it's effectiveness depends on the organisational culture of the FD and EMS.
[Just want to add, this is my personal experience, primarily drawn from working in an urban environment with a large department that didn't have a good working relationship, and a lot of historical animosity. Other's experiences may vary greatly.]