I would think heavy rescue vehicles would carry small pumps for special operations (such as a flood) They can still be a dry truck just with some support pumps for the other trucks.
Agreed, but to get to the 1500 GPM rating, they would have to carry at least 5 of them (presuming they all have an output of 300 GPM), taking up a lot of space and reducing the amount of gear the rescue could carry. Just keep in mind I haven't been on a fire truck in 30 years, and things may have changed just a little bit since then...