Re: default-truncate-lines

[Richard Stallman]

              /* If this variable is a default for something stored
                 in the buffer itself, such as default-fill-column,
                 find the buffers that don't have local values for it
                 and update them.  */

The reason for this is so that variables like `fill-column'
which are stored in slots in the buffer
can have buffer-local values in some buffers and not in others.
Setting the default value needs to store it into the proper
slot in buffers that don't have buffer-local bindings of the variable.

The code seems straightforward.  I don't see how use of
`default-value' could replace it.