ftpd reset restart position (REST) at any command

I try using mirror 2.9 (old, very old perl script) to mirror ftp site
in passive mode and see don't working restart transfer: server ignored
'REST pos' command.

This because in ftpcmd.y cleared restart_point:

cmd_list
        : /* empty */
        | cmd_list cmd
                {
                        if (fromname)
                                free(fromname);
                        fromname = NULL;
                        restart_point = 0;
                }
        | cmd_list rcmd
        ;

(cmd is any command execept RNFR and REST) and mirror in passive mode
used next command sequence:

REST pos
PASV
RETR file

RFC 3659 allow this, but not recomeded:

"The server-PI may react to a badly positioned REST command by ....
save the restart value and apply it to the next data transfer command
..."

Why this case (save the restart value) is bad choice?