A user will see no difference from a machine running ECC and one not running ECC. Would you see a difference between writing to a RAID 1 array or straight to the disk in real world usage ? Nope, you wouldn't either. Nor would you notice the dual power supplies in the system.
A professionnal running a long render or other time intensive and memory intensive application though doesn't want to see a "difference", he just wants extra assurance that if something does go wrong, he doesn't have to start over. Hence why Error correcting memory is important to him. These people don't care that you can price a 500$ equivalent system on NewEgg, they want reliability and failsafes in case of failures.
Also, professionnal grade hardware is garanteed to be in stock for replacements for a very, very, very long time. We have hardware here that is over 11 years old, and we can still get the parts for them. I don't mean newer parts, I mean the same part we got new out of the box in 1999.
This "value" is not in the actual specs of the machine, hence why they look overpriced to consumers who don't care about this professionnal level of value, just like they don't care about mirrored drives or power failures.