IDK, I've seen some odd RAM failure modes where the base OS would run fine, but then depending on the load (e.g. games) some weird stuff would happen like a BSOD after exactly <x> minutes of gameplay.

Once ZFS writes corrupted metadata with corrupted checksums to the disk, it's very hard to recover that data. Yes, ZFS is not backup, but the implications of serving clients messed up data is also worrying in certain business cases.

Given that ECC RAM isn't (much?) more expensive, why not just use ECC? (For my microserver I remember it being the same price as non-ECC.) Well, because CPUs with ECC support are rare, etc.