No real benefit to hard cases unless your tracks require it.
The theory is that hard cases will protect the cells. As long as your soft cases are secured properly in the battery box, I don't see any benefits to the hard case.
I actually don't like hard cases because I can't see and feel how my lipos are reacting. Lipos puff and get soft when they are going bad. You can't tell or see this happening very well with hard cases.
Plane and Heli guys have been using soft cases for years with no issues. They don't seem to have a problem with soft lipos in plane/heli crashes... i doubt our crashes are much worse.