Yes, I understand that. The reason to run memtest on ECC memory is to verify that every bit of memory is good before you finish building the system, installing

Yes, I understand that. The reason to run memtest on ECC memory is to verify that every bit of memory is good before you finish building the system, installing an OS, and putting it into production. If you have, say, 32GB of memory, there's no guarantee you'll hit those bad bits for ECC to log an error until you're doing something memory intensive two weeks later. The primary use case of ECC is to handle random bit flips from cosmic rays and whatnot, not to mitigate bad hardware. If you have bad memory, replace it, don't rely on ECC. The only way to test memory is to run something like memtest, where you read and write from every bit in memory. If you're OK with finding out a month later that ECC is logging a hundred uncorrectable errors an hour whenever you do anything memory intensive, then sure, don't bother memtesting. If you would rather deal with it beforehand, then run memtest.