wrong default for backup-by-copying-when-linked?

[Dave Love]

This can't be a new issue, but I don't recall discussion of it.  It
seems to me that backup-by-copying-when-linked should default to t, as
I actually thought it did.  I've just been bitten by breaking links to
some system configuration files.

Is there a good reason for the current default of nil?  If so, I think
it should at least be commented.