Comment 1 for bug 101301

Registring upgraders which you cannot use anyway should not be registered. This
indeed is a waste of ressources.

Registring at startup has a reason: third party products (i.e. SilvaDocument)
can register their upgraders. How could you do this at upgrade time? Start up is
the only 'easy' hook I know of. Although Silva has a list of Silva products,
which you could use on upgrade time. By the time implementing this using the
startup hook was the simplest thing to do.