If they really are as busy as you say they might do it to split the workload evenly. If you have 2 medics they can go call for call no matter what. B/P crew or even I/P crew can't do that. I think it's probably a county requirement though.
Denver Health Paramedic Division runs P/P almost exclusively. They have a few Bs and Is but if I remember correctly they're trying to get them through medic school and only have medics.
I've heard whispers of us going P/P but I don't think we have the budget for that and we definitely don't have the staffing.
Some states/counties say for an ambulance to be considered ALS it has to be dual medic. Michigan is like that in some areas if not all, Delaware requires two Medics to respond to every call that's triaged ALS or upgraded by BLS. Again, this would be where I put my money at.
those are the ones off the top of my head, after dislocating my shoulder...again...and being loaded on pain meds to reduce it and refusing to go to the ER. My girlfriend is angry.