Even if the bolts in the pan stay tight, those side brackets will flex.
Believe it or not, that cross brace will minimize the flex, saving the mounts and increasing the life of the bolts in the aluminum pan.
The technical reason is that a stiffer assembly will only go into resonant vibration at a higher frequency.
The cross brace makes the assembly stiffer, the natural frequency of the assembly is higher. Probably higher than 60 Hz, the vibration frequency of the engine at full speed.
