Why

Hydraulics allow for a simple drive train with hydraulic hoses
Integrates well with Power Cube and rest of GVCS ecology
Peak power of an extra 10kW can be attained readily for 100 lb of weight and $500 cost
See Hybrid Hydraulic System
Can be paralleled readily for extra power, not messy transmission complexities

Why Not

Use battery cranking amps to get 30 seconds of extra power for passing
With 25A for 240 minutes - [1] - or 100A for 1 hour best. At high crank, we can get 1000A for 6 minutes. Safe to do 1000A for 1 minute. Can possible use a smaller battery - standard car battery - [2] - 800 CCA, 1000 CCA.
15 mph extra speed to pass a car - is 22 fps - so 30 seconds gets a passing distance of 660 feet - which is plenty - given that the 2 second rule to following a car is generally accepted [3]
If going 30 mph - 2 second rule means you are 88 feet behind a car. If you accelerate to 45 to pass - it takes you 4 seconds to reach a car, one second to pass it, and then 4 more seconds to come back in. 10 seconds to pass. This is done comfortably within 30 seconds of cranking amps.
The battery can run a 12V peaking motor, driving another hydraulic pump - which can produce up to 12kw if we use 1000A cranking for 30 seconds. This power should be added in series to add to the speed of the car for short bursts.
This provides an easy solution to peaking power by using hydraulic-electric-gas hybrid strategy.