For the record, It seems we've missed investigating the actual '/usr/bin/login' binary until now. I've quickly looked at it and there's one (documented) difference spotted:
* the util-linux version does not support the -r flag. Here's the what login(1) manpage in util-linux says about it: [...] BUGS The undocumented BSD -r option is not supported. This may be required by some rlogind(8) programs. [...] Here's what the shadow login(1) manpage has to say about it: [...] SYNOPSIS [...] login [-p] -r host [...] OPTIONS [...] -r Perform autologin protocol for rlogin. The -r, -h and -f options are only used when login is invoked by root. [...] Question remains how widely used this feature is? Can we drop it? Regards, Andreas Henriksson