Soft packs are commonly used in airplanes and helicopters which do not subject the pack to the many bumps and jolts a car gives, except in crashes.

Many tracks require hard cases as a safety precaution so they don't get punctured but the case doesn't do anything to prevent overcharging and other abuse. ROAR requires hard cases at their sanctioned races.

Most tracks that I race at allow soft packs as long as they're properly handled and secured in the vehicle. I use soft 5S packs in my 1/8 buggy strapped in with four velcro straps with no issues.