It seems to me that you are still expected to develop your own custom rsync
configuration to mirror CPAN. Frankly, that's too much effort for an ordinary developer; it's easier to just use CPANPLUS, or suffer on when you're offline.
I think if there were HOWTOs for platforms, e.g. MacOS X, each BSD, each Linux distribution, more people would be mirroring CPAN (e.g. on their laptops).
The intersection set between Perl application developers and those who are willing to invest half a day in setting up a CPAN mirror may not be that large. I know that the payoff isn't good enough for me, but I would do it if it were cheaper in terms of effort.