For privacy.
If I have sufficiently many users, it becomes quite possible (with reasonably simple-to-set-up systems) to make it virtually impossible to reconstruct any user's complete clickstream.
With each machine broadcasting a publicly accessible IP address that includes the full MAC address, it becomes trivial to permanently track the full clickstream of every user on my network -- and even to track that user's clickstream between all of the networks that they ever participate in, since the (publicly-broadcast) MAC will still be quasi-unique.