Why replace when the system is stable? I guess there may be an increased chance of multibit errors. But sometimes new ram is flakey or disturbing the rack causes other problems.

Is ECC a crutch? Sure. But it's hard to walk with a bum leg/bad ram, so why not have it? (Cause it's expensive is a fine reason, but if it were closer to 25% more than 100% more, it'd be easier to say yes)

Memtest86 is great, but systems change and most people aren't running memtest frequently. On my non ecc systems, I run it during setup to make sure things are good, and only later if things get crashy... but if things get crashy because of bad ram, my data may already be corrupted.