A while ago, a couple of base packages were only available on ftp.gnuab.org and crosshurd needed ftp.gnuab.org to work properly. I haven't checked lately, perhaps matters have changed.
As I said, alpha.gnu.org in the good old days was just a temporary dump site for quick hacks. The less it is supported the better infact. On a "perfectly" working Debian system you should only need to have ftp.debian.org in your sources.list, nothing else.
