Negative pitch would work to regain rotor speed in case of a botched autorotation. The problem is what to do if somebody selects negative pitch while under power.
Easy you say? put a lock out in the lever such that it can't go past negative if the engine is running? Really?
Robbies still kill you if you forget to pull carb heat, when an automatic carb heat actuator could easily be fitted. Why the hell would we try to save somebody who can't keep their rotor speed in the green?