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Many times you don't want to be identified while surfing. For one thing your browsing history might be sold without your knowledge, and with no benefit to you.

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Related question on SuperUser: How to avoid exposing my MAC address when using IPv6? – grawity May 6 '11 at 20:45

Whether or not it is a 'major concern' is a point of opinion, but this idea has been had before, and resulted in action. Microsoft itself proposed an RFC to cover just this. RFC 4941, Privacy Extensions for Stateless Address Autoconfiguration in IPv6 (Sept, 2007). IIRC, Windows defaults to use this, the Linux IPv6 stack has this as an option (your distro-of-choice may already set it by default), and OS X 10.6 also has support. That's a very large portion of end-user devices right there.

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On most systems, these privacy extension can be enabled by running echo 1 > /proc/sys/net/ipv6/conf/all/use_tempaddr as root (for runtime configuration). – Lekensteyn May 6 '11 at 20:46
+1 The largest concern is tracking people as they move from one point to another and any valuable information you could extrapolate from that. – Chris S May 6 '11 at 20:50
Does anybody here know how to enable it under OSX? – Fubarro May 6 '11 at 21:00
@Fubarro The link @grawity posted has that information. – sysadmin1138 May 6 '11 at 21:10

Yes, that is a concern, and it is the reason some operating systems intentionally do not use the EUI-64 standard, preferring to use random data for the last 64 bits of their address.

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Not entirely random though. A few of the bits are used to indicate how the address was generated. – kasperd Mar 27 '15 at 13:41

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