Dirt Bikes and Rally Cars are driven by people in or on them.
R/C cars are driven from a drivers stand.
Full scale racing does not 100% translate into r/c racing.
Lighter is better to a point, and for the situation.
Removing rotating mass has a bigger effect than sprung weight. IMO
Removing weight to put it where you want is probably good. Some pro drivers run their batteries up front.
Ultimatlely we are tuning the chassis's we have to work with to get the most out of the design.
The wieght rules are designed for safety and to keep costs "reasonable".