the need for flexible chassis mounting is determined by the design of the truck. Some trucks don't have flexible chassis and therefore a flexible mounting is not required, those that flex need a flexible mounting.
Now, you can say that to some degree all chassis flex and this thefore reflex on ho ridged a non flexible mounting can be. some a fixed, some such as the KAT1 use spring loaded mounting, others such as the unimog use a three or four point pivot system. it really depends on how much you are going to challange your truck off-road and how ridged your rear fletbed or box is going to be to compensate.
All very complicated, if you have a truck in mind I can give some specific advice. give me a ring 07714 234733 if you want to discuss.