[lxgr]

It would also seem to break address privacy (usually not much of a concern if you authenticate yourself via SSH anyway, but still, it leaks your Ethernet or Wi-Fi interface's MAC address in many older setups).

[gspr]

This is a good argument for not making it the default, but it would be nice to have it as a command line switch.

[rnhmjoj]

Well, yss, but SSH is hardly ever anonymous and this could simply be a cli option.

[lxgr]

Not anonymous, but it's pretty unexpected for different servers with potentially different identities for each to learn your MAC address (if you're using the default EUI-64 method for SLAAC).