I'd argue not about security, but transparency - when having your mac address partially included in the IPv6, you would basically allow browsers and other systems identify you without additional steps.
Early IPv6 commonly used EUI-64 addressing, which did embed your MAC address into the IPv6 interface ID
Sure, but your MAC address is easily spoofed. In fact, all major operating systems do it nowadays for public WiFi systems and you have to explicitly opt-out of randomising your MAC Address when connecting.
