Obfuscation. By inspecting packets coming from my network now you can tell what MAC addresses are in my network and also internal network topology. It's part of the reason your cell phone feels the need to randomize its MAC.
only if you're using EUI-64, but I don't think many things use that anymore. I think the only thing is cisco shit. but even then I suspect that they have RFC7217 on by default at least.
"something ipv6 something something you may be vulnerable if you buy network gear from a normal vendor"
"Ok, let's not do that ipv6 thing"