As the prefixes routed by SixXS are all listed in whois, it would not make much difference. Note that "privacy extensions" are only useful when one is changing between prefixes. Thus lets say your computer's wireless has MAC 00:11:90:1e:e1:6e and thus gets Link-Local fe80::211:90ff:fe1e:e16e. Your home wireless is 2001:db8:2222::/48 and your friend's wireless is 2001:db8:aaaa::/48. If you are at home you will get 2001:db8:2222::211:90ff:fe1e:e16e, but if you are visiting your friend it will be 2001:db8:aaaa::211:90ff:fe1e:e16e and thus, as the EUI-64 portion stays the same remote services could use this to see that it is you again. In that situation privacy extensions may make sense. If you stay in the same network it does not matter. Note also that cookies, behaviour and logins to correlate the same though. IP addresses do not matter. See also How the Internet sees you along with the videos on youtube etc.