I'm not 100% sure why they have a "repo" for an "installer". It sort of rather defeats the purpose. If they're going to do that, just put the client in a DEB and put that in the repo. At this point I see no reason in adding the repo to apt. Either way, it'll get sorted, Steam for Linux is already quite great. Still can't get over that TF2 runs better in a window in Linux than I've ever had it run in Windows.
There is a launcher script that is installed in /usr/bin/steam. There was strong feedback from users that updating this script should belong to the system package manager. This is why the repo exists - so this script can be updated and managed by the system package manager.