Just to add, some late rmodel cases do come with plastic fittings, they may become brittle over a period of time, might get damaged or split or break if the PC case is mis-handled or dropped. Even some plain PC cases do tend to bend in ways when manually "booted" (with your boot!) and also do tend to get a rattle between the joint fittings or if the PSU is too big for is mounting and touches the inside part of the case (some have stick on tabs so that the PSU does not touch the inside area).
Some PC cases do tend to go rusty when subjected to water, dust accumulation can also make the tin of the PC case deteriorate, and some solvents used to erase marking pen markings on the case either on purpose or by a child, can cause discoloration and unsightly marks on the outer covering.