it's not at all unexpected that ifconfig croaks on things that aren't compatible with Linux 2.0 (NET-3, I think it was called). With the introduction of Linux 2.2 and all the new fancy network features it supported, the net-tools package (ifconfig, route, etc.) was deprecated in favour of iproute which supported all the new stuff. So I think the paragraph in the manual page is accurate, it's not a must unless you need to preserve compability with Linux 2.0. There's a host of other stuff that will only work on kernels 2.2 and above and that can only be controlled by the tools in the iproute2 package, for example "ip rule", "ip route [...] table NNN", and of course, these labels that makes use of the relaxed syntax rules.
I really wonder why so many distributions still ship the deprecated net-tools package. It hasn't seen developement in seven years... But I digress.
Anyway, I disagree with you in closing bug #55020, LaMont. These labeled addresses have been perfectly valid since 1999 (when Linux 2.2 was first released), and that Postfix and Nmap still croaks on encountering them should by now be considered a bug, in my opinion. Most other applications have no problem with such labels.