After enabling a new mirror (mb scan -e), it is not really enabled, because it
first needs to be marked "online" by the (separately running) mirrorprobe. This is
something that new users frequently run into.
The mb scan command could mark the mirror online right away. The only reason not
to do so is to prevent harm from typos, and from the mirror going offline.
If the mirrorprobe is already configured to run every minute, it would disable a
broken URL soon anyway.
Users may miss to set up the mirrorprobe in general; but forcing them there by the
above situation is not really nice.