Maybe the issue is that undetectable errors are possible, but if the system is in such a bad way that they're happening at any rate, you'll also be getting quite a lot of the detectable ones and then get prompt notice that something is wrong.

Whereas without ECC you could have silent data corruption for years and only discover it after it gets severe enough to warrant a manual investigation, after the damage has already propagated to your backups.

> Of course, the majority of desktops use non-ECC RAM and it's mostly fine, so I assume this is only ever going to matter in production workloads, and exactly what impact it has is hard to gauge.

There are two reasons it's useful. One is the cosmic ray random bit flip that happens even to hardware in good condition, and then ECC can usually detect and correct it, but that's less common and more important for production workloads.

The other is, your hardware is experiencing a higher than average number of random bit flips, and then ECC gives you immediate notice when this starts happening instead of letting it sow chaos until something crashes so hard you take notice.