At some point you need to have a device capable of switching or regulating the power from your batteries to your motors. Normally this is a MOSFET speed controller or weapon-firing relay. Both types are capable of failing in such a way that they will conduct power without any voltage supply to their control inputs, leaving you stuck in forward drive or with a flywheel stuck on.

You may find this risk acceptable, indeed it is fine by the current FRA rules, however, a separate power relay reduces this risk significantly.