Just a small-sample anecdote.
When I was a student and doing clinicals with [unnamed urban midwestern fire/rescue service], the FF/Medics all vastly preferred being on the pumper to being assigned to the ambulance. They seemed like cool guys, and they weren't derelicts on the ambulance, but they treated it as a chore, or just something they just sort of had to do to get to their next pumper shift and it showed in how they handled their calls. I don't know if that's indicative of how all FF/Medics approach the ambulance side, but if it is, I'd rather the services be separated.
I did my first five years a tiered single role EMS system. Back then, I was 100% about EMS, and had no doubt that I wanted to be on the street for 20-30 years doing ALS 911 txp. Then, the job started getting to me - having to post on street corners, no sleep or downtime, mandatory forced OT, constantly interrupted meals (sitting down for dinner three times and still not eating - that type of thing), and people calling for every little thing (missed lunch to run a call for a kid with head lice once), and most importantly, for not enough money. There's also the boredom/frustration factor that there's no realistic career ladder, so street EMS txp is really the only thing most of us could do for their entire career.
So, I went to fire based EMS. At that time, being a medic was what I still liked doing the most, but fire had interested me for some time. The real reason for the jump was because I needed to do something different than just ambulance txp day in and day out, with better benefits and pay. So, it becomes frustrating when you want to do more than just transport, but keep getting moved back to the ambulance repeatedly. After some time, you just get tired of wasting your time running non-acute calls, missing drills, meals, PT, and sleep. The engine is back in service in 20 minutes, but if you're transporting, you know that it's going to be an hour to an hour and a half until you can get back to the station, if you can avoid a second call on the road. I think that it's the lack of relative downtime that breeds resentment towards ambulance work. There are many other medics like me that started single role and then went fire. They all say that they would never go back to single role. They also all say that at one time they liked being medics, but that it was beat out of them by having to ride the ambulance most of the time, typically running people all day and night with minor issues that don't really need an emergency room. There's also the QA/QI thing - medics on the ambulance have much, much more to worry about than someone who rides backwards on the engine.
More or less, I feel the same way as these other dual role medics. I get a good ALS call once in a blue, and I truly enjoy those type of calls, but I have no use for the 90%+ mundane calls that keep us out of the station all day. As a result, I prefer to be off of the ambulance as much as possible, to do other things and once again look forward to coming to work. The good calls are too infrequent to make it worth it to do a tour on an ambulance - we're an all-ALS txp system, so we run everything. It's a lousy feeling to be taking a beating on the ambulance day in and day out when everyone else in the station gets to go back in service, enjoy downtime, and get things done throughout the day. I've seen some hardcore medics grow to resent the ambulance for these reasons.
Also, let's not forget that even in single role EMS, there are plenty of people that resent ambulance txp. EMS is a stepping stone job for a lot of people, because it pays well enough to pay for room and board while they work towards a better career. Average single role EMS burnout is 7-10 years for a reason. This resentment and burnout is hardly just a fire service thing. It's just that in the fire service, there's other paths to choose to get out of txp. Resentment develops when the employee is denied the opportunity to pursue those paths.