It doesn't really matter if the errors were caused by cosmic rays or the ram failing -- it's still dangerous and relatively undetectable.
I'm sure that this can happen to me, that in any particular moment, my memory can get corrupted by these rays, or something else, and then, the computer can misbehave or crash. And if I really wanted to be sure that it will not, I'd need to have some kind of protection against it. But compared to many other possible faults, is it really anything more than a very very rare and minor reason of a computer failure that I simply can just discard on my laptop, which is nowhere near a "critical and vital" system?