First, there is just the global distribution argument.
Then there's the dynamism/adaptation argument: a recent NYT story talk about how parts of the Brazilian church are "countering evangelicalism and secularism with livelier worship."
So both the center of global gravity in the church and its most likely form of marketing salvation.
Brazil is experiencing a huge expansion of its middle class. People undergoing such tremendous socio-economic churn want moral handholds. But they also want it in a form that they find conducive to their daily lives, and the traditional Catholic church has simply changed too slowly in response to the competition.
I saw a version of this in Ethiopia two years ago. Place is booming and all sorts of change happening. The classic Ethiopian Christian faith - very Catholic in form - just wasn't getting it done. But you'd see these evangelical churches (mostly Pentacostal) everywhere and they'd be packed (I mean, with crowds extending out into the street!) - and jumping.
My fear with Benedict is that he retires so he can - in his typical control-freak fashion - determine his successor. Let's hope it's something more than personal ego at work here.