The point is that you can't prevent failure by just buying something. You have to deal with the fact that failure can not be prevented.
In other words, if a single defective DIMM somewhere in your deployment is causing catastraphic failure, your mistake was not buying the wrong RAM modules. Your mistake was relying on a single point of failure for mission critical data.
People might not be preventing failure by using ECC, but they significantly decrease the likelihood of having to deal with hard-to-debug problems caused by bitflips.
