Well ECC is not the be-all end-all. There are several sources of bit errors. SD-Cards, USB Flash drives, TCP/IP Packets, HDDs, SSDs etc etc. Thats just the 'bits' you have control over. If you're copying data from somewhere else, you have no control over what the other person does to it, etc etc. I wont't produce a thesis here, but you get the idea :)

There is about a 1-2% performance overhead for ECC. Not that that many tasks are bound to memory I/O.