With separate pumps and wheel motors you can replace one that fails, with the single piece you are looking at one large expensive piece I beleive. Most all if not all the commercial duty ZTR I have seen have them separate.
The pumps and motors are roughly $4-500 each and I have to believe the single units are close to $1000 if not more.
I know on the Cub forum some of the single all in one units (not sure the brand) were failing at 2-300 hours or so and are $$ to replace.
I agree there is a reason for separating these components - sort of best of breed components for the job maybe? Other aspects Heat, Serviceability, adjustments, tuning, with the separate units you can use one pump and different motors depending on the size of the unit - more flexibility.