In "theory" it needs ECC because you must also protect the link between the CPU and the RAM. So with ECC fully in DRAM but no protection on the bus, you risk so

In "theory" it needs ECC because you must also protect the link between the CPU and the RAM. So with ECC fully in DRAM but no protection on the bus, you risk some errors during the transfer. However maybe this kind of errors are rare enough so that you would have less than one per century. It probably depends on the motherboard design and fabrication quality though, and the environment where it is used.